Keep it clean! | Yes, you do need to wash your make-up tools. Yes, really. I know it's boring. Do it anyway. Honestly, I find it makes such a difference to how my make-up applies - quite aside from the fact that better skin = better make-up, and not introducing unnecessary bacteria from dirty tools onto your skin, will give you better skin. I wash my Beauty Blender before every use (this is easy as I use it damp!) and try to wash my brushes at least once a week, although I aim to wash those used for liquid or cream products after every use.
Consider your base | Sometimes, for whatever reason, products just don't mix well; a certain primer can be brilliant under one foundation, and frankly awful under the next. If you find a foundation doesn't sit well on your skin, try it with different products underneath - that means moisturisers as well as primers. If you have dry skin, a good moisturiser will help prevent product clinging to any dry patches; for oily-skinned girls, a good primer can make the difference between two and ten hours wear.
Less Is More! | Applying too much product is a surefire way to end up with your make-up looking cakey and sliding around your face. Sure, we all want a decent coverage, but the best way is definitely to start with a little product and add more as and where you need it; it's way easier to pop a bit more on, than take it off again! Especially when trialling a new product, start with a small amount and add more if you need. All the setting powder in the world won't keep your base in place if you've used twice as much product as you need - you can consider that a bonus tip - always set liquid base products with a powder to prolong their wear!