I always stretch when working out. I'm convinced its saved my ass from many injuries, and 100% sure its prevented more lumbar strains. Heres what I usually do, with more emphasis on the bdpt being worked that particular day:
hams & lower back - normal hurdlers stretch (tight hams accounts for more lumbar strains than you'd imagine)
quads - hold on to a post or something that lets you squat down fully, then lean back far slowly - don't let the knees go out beyond the toes to prevent knee stress
lats - simply hang from a chinning bar
delts - same as above ^^^ - also stand with your back to a post, reach back and grab the post with arm straight and stretch - I also do alot of windmills to loosen up the entire shoulder area
biceps - stand with your back to a post, reach back and grab the post with arm straight and stretch
triceps - position the arm like your going to do an overhead db extension, then use other hand to grab the elbow and pull it towards your head
chest - facing the squat rack, put each hand on the opposing posts, keeping the feet stationary, lean forward slowly and stretch - like doing a db flye standing up