i am 31 years old and still struggle with acne. the doctor that gave me an acne prescription once in my teens said 'you'll grow out of it'. yeah........ not. i do have to say my skin isn't nearly as bad as it used to be. grade 9 was the worst - i had other kids in school that didn't even know me calling me 'chicken pox girl'.
don't wear makeup (foundation, concealer, etc). this stuff just clogs up the pores and makes acne worse. even those 'non clogging, hypoallergenic, etc' makeups are bad for your skin - trust me, i tried every single one when i was younger, in an effort to hide the zits.
salicylic acid and hydrogen peroxide never worked for me. those are the two main ingredients in most acne treatments available over the counter. both aggravated my acne and made it worse. nowadays i will occassionaly give my face a swipe with hydrogen peroxide on a makeup pad, and it helps to tone my skin, but i only do that every couple weeks or less.
tea tree oil works if your skin is not already oily. i found it didn't work at all on my face.
what works best for me is just to use a dove soap bar on my face once a day, and exfoliate once a week (with either st. ives apricot scrub or a plastic shower poofy with the dove bar soap). splashing my face with cold water throughout the day when it is humid helps my pores to stay clean and keeps them closed up more (i have very visible facial pores, yay). my acne definitely gets worse in the warmer months, that's for sure. winter isn't too bad, as our winters are dry up here, and it almost gets to the point where i have to use moisturizer in the winter, if i'm outside a lot.
don't overwash, as this will aggravate your skin to produce more oil and your pores to produce more sebum (this is the stuff that causes zits), producing more acne. once a day with soap, and clear cold water splashes should help. exfoliate with a gentle product, and if you find an acne product that works well, only use it once a day, as overuse can cause your skin to produce more oil, and your pores to produce more sebum, to 'protect' itself.