1. Leave the make-up at home. This is one of my personal pet peeves. I cannot comprehend women who wear a full face of make-up to the gym. Nobody there is looking at you or caring if you look haggered. You're more likely to attract negative attention for being a vain prima donna than looking a bit worse for wear; at least if you look like crap while you're training people will think that you take it seriously! But, aside from the idiocy of it all, make-up+sweat=blocked pores. And, not only will it block your pores but you'll be left with little channels down your face that the rivulets of sweat have caused...so sexy! If you really need to cover up a few bits and bobs, wear a light weight, liquid concealer and maybe some mascara but, honestly, it's not necessary when in the gym.
2. Don't moisturise. And, by this, I mean don't moisturise too close to working out. You need to let the cream have time to absorb. If you apply straight before you start exercising, you'll sweat it all out and be left with a greasy, slimeball of a visage!
3. Drink water. Not only is it important to stay hydrated when exercising but the more water you take in, the more you sweat meaning the more impurities that are leaving your skin. Win all round.
4. Have a kit. If you're on the go and work out coming from work or college, have a little pouch of just a few travel-sized bits that you can clean your face with before you exercise. I just have some make-up wipes and little bottle of Simple cleanser. I also carry a small tube of moisturiser for around my eyes because the wipes dry that area out.
As you can see, there isn't a whole lot you need to do to keep your skin healthy in the gym. The most important point is to go with a relatively clean face so that nothing is sinking into open pores that shouldn't be. Ever since I started removing my make-up prior to working out, I haven't had anything like the bumps that used to pop up after a week of training so it really does work. Hope this helped some of you!