My favorite method for figuring out the typos I want is to close my eyes, and type the domain name as fast as I can about 25 times or so. Whichever mistakes I make the most - I go research that typo, and if it makes sense and is available, I buy it.
(And I type 120wpm with very few typos, so it has to be a pretty good one. However, there are a few words that I almost *never" type correctly the first time)