Foundations can really be a miracle if it's used correctly. Now, I've seen it used incorrectly many times. That is to say not that there's one correct way to apply foundations - there's just certain things that must be avoided and things to consider when you buy and apply foundations. These are in no particular order but are problems I've encountered and seen people around me encounter.
1. Never buy anything unless you've tried it on → let some one at a makeup counter apply your foundation for you and match your skin tone. They know the products best, after all. More than once I've seen friends chose a color they think is closest and find out that it's too dark/light/grey/yellow/salmon when they get home. I usually try something on, wear it for a day and make a decision by the end of the night if I'm going to go back and buy the product.
2. BLEND → There's so many times when I catch girls with foundation on that stops at their chin so it looks like they are wearing a mask. Make sure you apply your foundation down to your neck, around your ears, and up to the hairline. There's nothing more embarrassing than having one of your guy friends or your boss make a comment about how they can see where your foundation ends, trust me :(
3. Set your foundation → Or else you'll find yourself with half of it missing in a couple hours. Now, this is always something I find a bit hard to decide on --- at least when it comes to what to use. Sometimes I'll go with a translucent powder but if I'm wearing a foundation that's too light I'll use a MSF by MAC to even out the color a little bit.
4. If you're using dark colored eyeshadow - do foundation last → I've walked around all day with fall out under my eyes and haven't noticed. There's really a simple way of avoiding that.
5. Using your hands is okay → As long as you wash them! This is my favorite way to apply my makeup - I only use my hands when I am working with my own foundation though. It warms up the product and makes it early easy to blend! So, the problem of streaks and not being able to get your brush into the corners of your nose and in other "hard to reach" parts of the face is no longer a problem.
6. PREP YOUR FACE → Always wash your face and moisturize. I think it's really easy to tell when some one hasn't moisturized before putting on foundation because you can see where it gets dry and where it flakes off their skin - it's never attractive.
7. You can always add more → I suggest starting out with a small amount of product when you switch foundation. Apply a small amount and if it's too sheer add more to add coverage - you can't go back if you put too much product on your face and end up looking like you're face is wet. There is a different between too much product on and a "dewy" finish.
That's all I can think of so far! I hope it helps you guys --- I would have loved to learn a few of these things without having to experience them when I started out with foundation. I don't think they're all that complicated and amazing things that are secrets of the universe - but I think that they are helpful to know. Hope this helped :*