The only thing I can say is DONT GIVE UP. It can seem impossible, but with time it becomes second nature. I don't know anyone who had a tougher time with chord changes than I did. Most people I know said they got them down fairly well in about a week. It took me nearly a month. But none the less, i do it without thinking about it now for the most part.

It can be frustrating, but its never impossible. What helped me the most were practicing scales, and the song Wonderwall by Oasis. Even if you dont plan to play lead, the scales, and the finger exercise Lisa linked, if played properly (IE all 4 fingers) were great to develop finger independence. Wonderwall, on the other hand, is great strumming practice, and your pinky/ring finger stay on the same two frets/strings the entire time. Not only does it keep you from getting bored by giving you a song to play before you get your full open chord changes down, it's a great song to practice strumming. Sounds good played slow or fast.

At least those are what helped me the most. Your mileage may vary. The point is to stick with it. Everything comes around eventually :D