if you're currently in school, i say ask for the LoR when you feel that your grades and relationship w/ the prof is at its best/peak.
if you've been out of school for a while like i have, just make sure your manager/super/boss has ample time to write you a good letter and not rush through it.
and make sure to check the supplemental apps for addtional LoR or recommendation forms. i wasn't aware of some recommendation requirements of a local school and ended up scrambing to get them all done in time.
as for the pers statement, it took me A LOT longer than i originally anticipated, so it's not a bad idea to start early. i must have written 5 different versions before i picked one.