1 - practice slowly. Go in extreme slow motion a few times and make sure that every finger is going exactly where it needs to go with no wasted movement. Look for pivot fingers - the fingers that stay in the same place between two chords - and practice not moving them. Get it clean. Make sure your hand is doing exactly what it needs to every time.
2 - Practice changing in time. I've seen a lot of people just get into the habit of stopping when they change chords when they're learning to play. They hit a point where they know the chords, their fingers are moving well, and they are stopping purely from habit, not because they actually need to. To break that (or avoid it in the first place) choose the easiest two chords you know - the ones with the least actual movement between them - and practice going between those two without stopping. Strum one and keep a steady beat. Keep the tempo slow to start with. The key is that it's steady. Strum that chord and count to four then change to the other chord without missing a beat and keep strumming it. After four change back. Start with a slow tempo so you can get used to getting it and then once you have it gradually speed up the tempo. Repeat with other chords.