I'd have to echo the "practice" advice on this one. There's no better way to build calluses than the old fashioned one. But that doesn't mean you have to punish yourself, either. If you haven't played in awhile, you'll want new strings anyway, so think about going to a lighter string gauge - even extra lights. (John Pearse, may he rest in peace, recently developed "slightly light" strings, which are a good bet if you don't want to go that far. You can still find them in better music stores.)
Checking the setup on the guitar is a good bet too.