well man,
practice practice practice is pretty much right; however, You might wanna check the way youre holding the neck of the guitar, you need to have the joint of your thumb on the middle of the neck so you can curve your finger around the front of it to properly press the strings down. It really takes time, find a friend that plays guitar and ask him/her how to get your hands around it right, it's easier to be shown then told. You have to learn to reach and train your fingers to stay up over the other strings and still press down on others. A good chord to practice that with is a G. As for an A, it's hard for a person with big fingers, instead of using your ring, middle, and index fingers all in a row, try putting your middle finger on the D string, then your index finger on the A string, and your ring finger on the B string. That is the actual proper way to do it, but a lot of people put their middle finger on the A string and their ring on the B, and the index on the D. I hope you understand what I'm sayin. You really have to squeeze them in. The most important part is to make sure you're curving your fingers around rather than letting htem lay flat. HAving them lay flat will make open string go dead and reallly ruins a song. I know you've heard it before, but make sure you practice a lot, also try putting your fingers on one at a time and not all at once, then pick your strings one at a time rather then strumming, this will help you make sure you're doing it right. I hope I've helped you in some way shape or form, so if you have any more problems post another message or e-mail me and I'll try to better explain it. Good luck and keep playing.