Here's a topic that I'm not sure people talk about quite enough.
But think about it: there's a REASON that runners wear spandex, and there's a reason that weight-lifters wear tank tops.
What might those reasons be? Let's explore this a little in conjunction with building muscle.
First of all, I would never recommend wearing really tightly-fitting clothes on your upper-body limbs while lifting weights.
Why? Because your muscles need to be able to breathe, and by "breathe", I mean they're going to swell up while training (if you're doing it right).
If there's nowhere for your muscles to go when they swell, this could cause some discomfort.
You're going to need all of your attention focused on building muscle, not on shifting around trying to get comfortable.
But you don't want your shirt to be too loose, either; you certainly wouldn't want it to get caught on something and have you stuck somewhere, not to mention tearing it.
I recommend an easy-fitting, dark-colored tank top.
That way, when you sweat, it won't show up as much.
Shoes can play an important role, too - especially when you're building muscle in your legs.
If you're doing squats, you don't want an old pair of shoes with no traction; you could slip, and that means disaster! The bottom line really is to be comfortable, and to wear what you think will make you look your best.
We're all humans, and one of the reasons we even TRY to build muscles is so we can look better.
So it makes total sense to look good WHILE we're training, right?