Get a metronome. It will be your best friend and worst enemy at the same time. Play the rhythm you want at some moderate to slow speed. It doesn't matter what it is, but I'm going to use 80 beats per minute (bpm) as an example. That's a nice slow-to-moderate speed.
So, play the strum pattern you want EXACTLY RIGHT at that speed. If you can't get the strum pattern EXACTLY RIGHT at that speed, slow down until you do get it exactly right. Practice at that speed for as long as it takes for the strum pattern to become second nature. Then speed up the metronome. For example, if you started it at 80 bpm, bump it up to 90. Strum it a few times, and you'll have it exactly right and realize that 90 bpm is ridiculous and why did Zurf ever tell you to bump it up so little. Then take it to 140 bpm. It'll stink. You'll hate it. You'll want to throw your pick out the window. Instead, slow it back down to 90 bpm where you have it exactly right. Get it exactly right a few times at 90 bpm, then bump it to 100, and then 120, and so on.
The idea isn't to practice it at speed UNTIL you get it right, because you never will. The idea is to practice it right at as slow of a speed as you have to go to get it right, and then get the speed up. In other words, practice it so that you can't get it wrong, rather than having to work to get it right.
I wish I was wise enough to take my own advice. It's really good.
Granted B chord amnesty by King of the Mutants (Long live the king).
If it comes from the heart and you add a few beers... it'll be awesome! - Mekidsmom
When in doubt ... hats. - B.G. Dude