I like best the suggestion to stop the girl well in advance of cumming and work on her for a few minutes and then go back. The only issue there is the blowjob - if you stop her from giving a blow job then you have to get her to start back up again, heh.
A slower pace is probably a good solution too, though it seems like it'd be pretty awesome to be able to get an enthusiastic porn star blow job for a good 5 minutes before release.
I can see where you might be able to control it a bit mentally, but surely this can be done without thinking of unappealing stuff. Maybe some kind of meditation or breathing techniques?
Otherwise I think the best method is just to have as much sex with as many women as possible and then you get the benefits of both practice and desensitization.