Ingredients: Really do your research on common foundation additives. I highly suggest the book "Don't go to the make-up counter without me." I was only able to read the first few chapters (Nook sample) but even those were incredibly helpful. It helped me to understand very common, yet highly agitating and damaging, additives in all types of cosmetics, and skin care products. Also if you are trying many different types or brands of foundations try comparing the ingredients of each formula; especially if you find that one is working better than another. I know that it can be tedious looking at the long and painfully tiny print of the ingredients list, but it can really help you to avoid certain things in the future. For instance, lets say your using Bare Minerals and you find that when you use it you often develop some form of skin irritation. In this case I would say that you are probably having an allergic reaction to the Bismuth Oxychloride in this particular foundation. Having this in mind, you would be well advised to keep away from foundations with this in it. I know that it may seem like a large task, but in the end it will be well worth it. Because healthy skin means that you won't have to use as much foundation.
Forms of Foundation: There are so many types of foundations and cover ups. The most common being liquid, followed by mineral, powder, cream, mousse, tinted moisturizers, and BB creams. If your having trouble deciding what kind of foundation to use, the first thing you should take into consideration is your skin type. For instance, I used to think that I had oily skin, I only recently realized that I actually have combination to oily skin. If your skin type is similar to this, than you can use pretty much any type of foundation. If your skin is dry, then I would suggest a moisturizing foundation, tinted moisturizer, or BB cream. And for those of you who have true oily skin, I would suggest staying within the mineral to powder range, as these will help keep the oil at bay. For combination skin, I would really encourage you to experiment with all different kinds of foundations to see which works best for you. A little side note; If you have dry, normal, or combination skin that tends to have acne, scars, or general unevenness, I would really, really suggest a BB (blemish/beblesh balm) Most of them have all the coverage of a foundation, and are very beneficial to the skin (for more information see my post on BB creams)
In Conclusion: Remember everyone's skin is different and what works for other people with your skin type may not work for you. Play around, do some research, find what works for you and what makes you feel confident. Good luck :)