[Phil Van Sexton]

It is important to raise with JJ to eliminate hands like A3 or KQ. JJ is the 5th best hand in holdem, and you'll usually be re-raised by anyone holding the 4 better hands. Therefore, you can be pretty confident you have the best hand pre-flop if nobody re-raises you.

Harrington on Holdem has several examples of JJ and TT, and he always recommends raising to eliminate overcards.

If in bad position or after 1 or 2 limpers, I'd make a bigger raise (ie 5-6xBB) to make sure I'm not going to be playing a big multiway pot.

After several limpers, calling is fine if you don't think raising will thin the field sufficiently, especially if you are in the blinds (ie really bad position). Against 4+ players, you can just play for set value.

If you raised pre-flop, I would bet out on 90% of the flops that might come down.