Ask them early so they know before hand that you want a letter and then when the time comes around, give them whatever they need in order to fill it out. Also, different schools have different supplemental letters of rec forms that they want filled out independently from PharmCAS. Also, give them addressed/stamped letters; make sure that you get everything to them once so it smoothes out the process. Professors are busy and want everything finished at once and the less of the hassle, the better.