Always use acetone to remove glitter nail polish - doing it with anything else takes so long you can probably grow new nails in the time it takes to remove it. I spent a long time using 'nail friendly' acetone-free nail polish remover and wondered why it took me about an hour to have any effect. Acetone makes it much easier!
Expensive doesn't always mean better - there are some great budget ranges around, and for many years I always thought high-end equaled high quality. Sometimes it does, but very often the high street has just as good (if not very similar) products. Some of my favourite products come from Aldi, Bourgois, Sleek and Barry M to name a few.
What you eat shows up in your skin - I wish this wasn't true, but what goes in to your body really does make a difference to your skin. Too many bars of chocolate, fried food and lots of lovely carbs leaves my skin looking tired and dull. It's the biggest organ on your body, so it makes sense that what goes in, must come out and if you're having problems with your skin, changing your diet can often have more impact than any product you can buy.
Don't be afraid to experiment! - whether with hair colour, hair style or makeup, live a little and try new things. I'm still not as experimental wit makeup colours as I'd like, but I'm a lot better than 10 years ago when the only eye shadow I owned was gold...
What beauty lesson do you wish you'd learnt earlier?