If you're getting a facial, always check out the skin of the person who'll be treating you. If they have bad skin, don't make the appointment! Generally, facialists use their own products, if they don't work for them, they're unlikely to work for you.
Never accept recommendations
Don't use cheap black hair dye if you're over the age of 25.
If you use those liquid cheekstains (Benetint, etc) you'll find them easier to apply if you dot the product on the finger you use to blend it, rather than straight on the cheek itself.
To find your perfect lip colour, try all your lipsticks out when not wearing any other make up at all. The one that still suits your naked skin is your ideal colour. There will be one, and the results will surprise you. Don't let it stop you wearing other colours though.
Colours from the opposite end of the spectrum will work best for bringing out your eye colour. If your eyes are hazel or green, try purples and violets. Blue and grey eyes should try browns, and brown eyed people should try greys. Nudes, or shimmering metallics will work well on all people, usually.
Always buy the most expensive foundation you can afford. More expensive formulations contain higher densities of pigment, and will cover better. It's the staple of your make up bag, so the better it looks, the better you look.
If you want to try a colour and you're not sure it'll suit you, buy it from one of the cheaper ranges, then if it doesn't suit you, you haven't wasted too much cash.
Pencils will sharpen better if you pop them into the fridge for half an hour or so first.
Eyecream, used straight from the fridge, will make your eyes much less puffy than if it's used at room temperature. Keep your nail varnish in there too, it lasts longer.
Soap, if stored in the airing cupboard before use, will last longer for some reason.
If you use the hot cloth method to clean your face, be sure to give your lips a tiny scrub too, it'll get rid of any scaly bits. Apply lipbalm straight after though.
ALWAYS exfoliate before using fake tan. And, if people start asking you all of a sudden if you've started using fake tan, then you know you've overdone it.
If you're not sure about a purchase, don't make it there and then. Go away and do something else for a couple of hours. If it doesn't come back to your thoughts by the next day, you didn't really want it in the first place.