Don't be afraid to pay for your polish. Sure you can find polish for less then $1, but this is one area that you often find quality in the price. This doesn't mean you need to buy a $10 bottle of polish for it to be good. However, be willing to spend more than a dollar on it as well. Sometimes, you really do get what you pay for. Stick with brands you trust. Oftentimes, it is a good idea to stick with brands you trust. If they generally have high quality products, then you will most likely find that you like what you get. Sometimes you can try something new and see how you like it. If it is great, then you can add that brand to your buy list. On the other hand, if it is horrible, you can skip out and not buy additional colors from them. Try it before you buy it. You can shake up the polish, open it and get a good idea of how it is going to look. Sometimes polish looks bright and exciting till you pull out the brush and find that the polish is actually thin and dull. You can also use your fingernail (or your grocery list) as a good place to give it a quick try. You shouldn't paint all your nails while picking out your favorite colors there in the polish aisle, but you can do a small test.
Choose colors that meet your needs. It is true that you may love the black or the yellow, but you do need to make sure that you will actually wear it. What will you wear it with? When will you wear it? Is it going to be something you can wear to work? These are all important questions to ask yourself. You don't want to spend money on something you aren't really going to use or that you will wear one time and then never wear again.