Also, a very firm, deliberate shot aimed right at an opponent can force a weak reaction volley that may produce a short sitter for your team. The pros are better than us mere mortals at hitting those sorts of strokes, but "testing" the reaction skills of your opponents is a smart tactic. If their reaction volleys are weak, you can make them more defensive, even when they've moved forward.
I also like the mindset of looking to be the team that hits down. Keeping the ball low to your opponents in a doubles setting will eventually force them to hit up and that's when you can truly hit through the other guys.