When you have a substantial chip lead...I am of the belief that it is time to take over the game. This doesn't mean loosen up to the point where you're playing like a maniac.

Raise in the same situations you normally would, only increase the size of your standard raise. If you normally raise 3X the BB....up it to 4X. Second....raise with the hands you would normally limp with (but please be aware of your position).

Just my two cents.