I usually fire a second and third barrell with air on the turn and river (that is If I observed them enough to be just floating) when I have Position. By the time the river comes and they check again...its going to be hard for them to call a pot size bet with a marginal hand.

When I'm OOP, and I'm against a weak player, I'll check-raise if I led preflop. If my opp. calls the flop bet, I'll check the turn, and if he checks too, i'll fire a pot size bet on the river. The only reason why I check-raised on the flop with air is to get enough money into the pot when the river hits. Usually by then the pot will be big enough for me to bet it's size and get my opp. to fold a marginal hand.