The best beauty advice I ever received/learned was...

As someone mentioned already... use a primer (especially for eyes). It makes a BIG difference in how long the eyeshadow stays on, and it also intensifies colors so you don't need to use as much eyeshadow.

Also... double tap your eyeshadow brushes before you apply glittery eyeshadow. I never used to tap at all because I didn't want to waste it, but I realized tapping is better than getting glitter in your eyes (oh the pain...)
 
not that I actually learned this but the best advice I'd ever give a young girl is to NOT get into the habit of picking your face blemishes. just don't, leave them alone. every zit I've ever had has gotten much worse because I pick and squeeze, and I have lots of scarring from it.

just never become a picker. don't do it.
 
I got these tips off other people or online and they've worked well for me:

Letting your facial cleanser sit for a bit longer before washing it all off.
Double-washing your face, maybe even triple-washing after a heavy makeup day.
^I do both of those nowadays and I feel like I'm going to bed with a much fresher face than when I didn't cleanse that way.

Also, exfoliating to help pimple scars fade. I had pretty bad skin in my teens - not terrible acne or anything, but just constant breakouts - and now in my early 20s I still have some minor scars even though I didn't pick/squeeze. I noticed a difference almost immediately after I started working at the scars more consistently with exfoliator.
 
I got these tips off other people or online and they've worked well for me:

Letting your facial cleanser sit for a bit longer before washing it all off.
Double-washing your face, maybe even triple-washing after a heavy makeup day.
^I do both of those nowadays and I feel like I'm going to bed with a much fresher face than when I didn't cleanse that way.

Also, exfoliating to help pimple scars fade. I had pretty bad skin in my teens - not terrible acne or anything, but just constant breakouts - and now in my early 20s I still have some minor scars even though I didn't pick/squeeze. I noticed a difference almost immediately after I started working at the scars more consistently with exfoliator.

+1 on letting your facial cleanser sit for a while on your skin before washing it off. It works a million times better that way!
 
This is the best advice I've learned and pass onto my mum and sisters:
Take off make up at night, even if you are drunk and tired use the wipes on your cheeks where pores can enlarge.
Pat eye cream on, don't drag.
Put cocoa butter on your body after showering.
Massage your facial moisturiser on.
Moisturise your neck.
A serum of whatever product range you use always has the highest potency so buy the serum if you cannot afford the moisturiser.
Girls with weaves/extensions - always 'oil' your scalp.
Wear a silk scarf at night to protect the hairline, prolong blow dries and keep hair silky.