I combine a number of factors. By the "community wiki rules" I ought to put them one-per-post.
Journal citation index. There are two versions of this: one by ISI and one by the AMS. I use both.
Why do I use them? Because my main reason for selecting a journal is that it gives an indication of the relative importance of the paper to someone who isn't going to read or be able to understand the paper itself. I fully expect a mathematician working in roughly the same area as myself to be able to make a reasonable assessment of my work without paying any attention whatsoever to the journal it appears in. However, from time to time someone completely incapable of doing that needs to get a vague idea of how well I'm doing. At the moment, one of the measures that they use is citation indexes so there is a strong incentive for me to get my papers into journals with the best citation indexes.
I know that citation indexes are unpopular so this answer may well end up at the bottom of the heap! However, I would like to challenge anyone who votes this answer down to put up a replacement that achieves the same end.
