If you don't have a spotter handy—or if flat bench hurts your shoulders—then the Smith machine decline press is one of the most effective Smith machine chest exercises that you can perform to add mass to your pecs.
And you'll soon find out why.
The ability to choose your own backrest angle is what makes decline Smith presses so effective for building the chest.
However, since most people are so used settling for whatever angle the fixed decline bench in their gym throws at them, they simply copy the setup for the Smith machine version. This blasé approach to training is a recipe for disaster as far as your chest development is concerned.
While steeper declines do indeed enable you to lift heavier weights, this is only because the range of motion (ROM) is significantly shorter. So you're not actually getting any more chest stimulation—only extra joint stress.
Plus, with excessive declines, all of your blood rushes to your head, which isn't exactly pleasurable.
The decline Smith machine press exists for one reason and one reason only: so that you may build meatier a lower chest.
It's not a powerlifting movement. Attempting new one-rep maxes on the decline press is a futile endeavour that only leads to shoulder problems and an ego that's far bigger than your actual muscles.
Also, since the decline machine press naturally has a shorter ROM than a flat press, you need to perform either more reps or more sets to get the same time under tension. So stick to sets 8-12 for the bulk of your training. Otherwise, you'll be leaving gains on the table.
Using a generic weight bench that doesn't have leg supports is one of the biggest (and most dangerous) decline bench press mistakes.
Couple this with an excessively steep decline, and you've got yourself the recipe for a serious injury—and I'm not talking about a cute little rotator cuff strain. I'm on about getting crushed by a loaded barbell, which is perhaps the most common cause of gym fatalities.
Fixed decline benches have leg supports for a reason, and that's to keep you safe. So please, unless your gym has an adjustable decline bench with leg supports, do yourself a favour and stick to the free weight version, ok?
Since you don't have to stabilise the bar, you can naturally lift a lot of weight when you perform the decline bench press Smith machine style. Of course, this is brilliant for building a bigger chest in less time. However, the extra loading can wreak havoc on your wrists if you're not careful.
So ever since I started getting wrist pain, I've made it a point to never bench press without my Beast Gear Wrist Wraps.
These wraps provide unwavering wrist support during all of my sets thanks to their thick cotton construction that prevets dangerous wrist hyperextension. I was worried initially by how sturdy the support was because I wasn't sure if I'd be able to bend my wrists.
However, since Beat Gear was smart enough to equip their wraps with elastic, it was actually ridiculously easy for me to get a full range of motion on all of my pressing exercises. I didn't feel restricted whatsoever, and I'm chuffed with the price that I got them for—highly recommended if you want to protect your wrists without spending a fortune.
Going gloveless may well make you "hardcore" by some people's standards, but it certainly won't make you stronger. If anything, you'll probably be weaker if you lift without gloves (or chalk) because your grip won't be as firm.
Not only do lifting gloves (good ones) prevent callouses, but they also enhance your grip significantly because your sweat can't seep through to the bar and make it slippery.
However, I just use this liquid chalk most of the time since I train at home.
That said, I know that many gyms don't allow chalk, which was the case for my old gym. So I used to wear these Grebarley Gym Gloves whenever I trained chest or shoulders (for back workouts I use Versa Gripps).
Anyway, I used them for about 9 months, and they didn't degrade in quality at all. I'm guessing that this is because of their durable microfiber construction. But who knows, maybe I just wasn't benching heavy enough (120kg) to tear them?
Ok, I've tried dozens of different pre-workouts. And while some of them give me an instant boost, most of them don't do squat for my high caffeine tolerance.
Anyway, the point is that no pre-workout, creatine supplement or anything else, has done as much for my bench press strength as the Mark Bell Sling Shot.
I mainly used it on the flat bench, but I've had great results from using it on decline presses too.
Anyway, after using my Sling Shot for 8 weeks, my bench press 1RM shot up by 7.5kg. And that's just as a result of the Sling Shot training. While wearing it, my 1RM is a full 22.5kg higher. And guess what that means?
A full 20 kilos of extra resistance for my chest and turbo-charged muscle growth. That's what.
I'm glad I train at home because frankly, I don't want the whole world to know about this bench press secret. Obviously, though, I'm willing to share the benefits of the Mark Bell Sling Shot with you guys. But let's keep our advantage by not telling every Tom, Dick and Harry about it, ok?
Performing the decline chest press Smith machine style is a great way to separate your lower pecs from your upper abs because you can focus on feeling the working muscle stretch and contract. It gives your chest that "lifted up" look that so many fitness models and bodybuilders have.
You can take some of the tension off your triceps and place it onto your chest by widening your grip. However, keep it within reason. Gripping the bar too widely reduces your ROM (and thereby chest development) and also strains your rotator cuffs. So, as always, balance is the key.
Your front delts are less active during a decline Smith machine bench press than during a flat press because your chest is in a stronger pushing position. Therefore, it naturally takes over the reins and enjoys most of the resistance.
You know what's funny about the flat bench press?
It's not even a real flat bench press—yeah, I'm serious.
Since most people need to excessively arch their spine to avoid straining their shoulders during a so-called "flat" press, they naturally turn the movement into a decline press. In fact, if you did a true flat bench press with no arch, then you'd wreck your rotator cuffs in a matter of weeks.
But, by doing the Smith machine decline press, you can get away with a smaller lower back arch, which is much healthier for your spine in the context of lifting heavy weights .
If I had a quid for every time I heard someone say that flat bench hurts their shoulders, I'd be sipping my post-workout shakes from antique glassware.
But if I had a quid for every time I heard someone say that decline bench hurts their shoulders, then well, I'd still be drinking my protein shakes from shaker bottles.
You see, with the Smith machine decline press, your chest does most of the work. Of course, this is just brilliant for building the lower pecs. However, it also means that your rotator cuffs stay healthier because your shoulders aren't forced way behind your torso (as they often are in a flat press). 
I won't deny that the decline barbell press is a phenomenal chest builder. However, once you start pushing weights that exceed your bodyweight, the lift-off portion of the rep quickly becomes impractical (and very dangerous) without a spotter .
Because let's not sugar coat it. You could drop the bar on your chest, neck or head and die. Which, unfortunately, still happens every year. Why?
Well, when you reach behind your head for the bar, you naturally place your chest and shoulders in extremely weak positions. As a result, they can't contribute much force towards the unracking process, which forces your triceps and rotator cuffs to pick up the slack.
Obviously, when two of your main pushing muscles are mostly out of the equation, unracking the barbell unassisted quickly becomes hazardous.
However, when you perform the decline bench Smith machine style, you don't have to reach behind your head to unrack the bar. Instead, you can simply rotate your wrists and start pressing. That's all there is to it.
And besides using a Hammer Strength machine, it's by far the safest and effective way to bulk up your lower chest.
The upper chest gets all glory now that we're in the "aesthetics" era. However, without enough lower chest mass, your pecs are never going to look big and muscular.
So it follows that the exercise that's best for lower chest development is also the exercise that's best for overall chest development. And according to research, it's the decline bench press that activates the most lower chest muscle fibres  .
Of course, muscle activation doesn't necessarily equate with muscle growth. However, this study backs up what bodybuilders have been saying and experiencing for years—the decline press is the best bench press variation for building lower chest mass.
If your gym doesn't have an adjustable decline bench, then the flat Smith machine bench press is a great replacement. It's one of my go-to chest builders because the setup is just so stupid-simple. Plus, I usually train alone, and there's just no way that I'm willing to risk dropping a barbell on myself.
The Smith machine incline press is a ridiculously-effective Smith machine decline bench press alternative if you've got a lagging upper chest. I switched over from the barbell version a few years ago, and I haven't looked back since because I saw near-immediate gains in my pecs. It's one of the most underrated chest exercises, in my opinion.
The Smith machine decline press is quite possibly the all-time best chest builder. However, a big chest is nothing without a strong set of arms to go with it, which is why I always perform the close grip smith machine bench press in my arm workouts. I personally superset it with dumbbell curls to save time—and because who doesn't love getting double the pump?