Originally Posted by: KirkD

I've done this a few times and for most of the basic progressions I've tried, I usually can do 60 bpm within a day or two using straight strumming.

-Kirk

What Kirk eluded to, but didn't outright say is "use a metronome." Start slow and go only as fast as you're comfortable with and then slowly turn the speed up. There should be plenty of free software based metronomes out there (if there's not one already on this site).

I know when I started playing I really wanted to play Nutshell by AIC, but couldn't get the chord progression to save my life. I played it over and over again until the fingers and wrist knew what to do next and about a month later I found I had to start slowing down because I was changing chords too fast!
Good luck and keep practicing.