Play games, analyse games, solve tactics. Pretty standard boring reply oh well :)
Ok... something more specific... hmm...
When solving a tactic don't just look for your move... to see if it works you have to find your opponent's best try for a defense. So try to use your "solving" mindset twice. First for your intended move, then stop and look around at all your opponent's pieces, now your his advocate trying to disprove that move.
That can take some effort and not easy (or impossible) to do for fast game and timed tactics.
As this mindset slowly seeps into your standard games, you'll be playing a lot better because this exercise does two things for you actually. You're looking for (what I call) your opponent's "most annoying" reply to your intended move, and by doing the tactics untimed your exercising your visualization ability.
If you're only interested in online chess where you can make use of the analysis board, I think it's still useful. The variations you look at will be more focused instead of looking at all sorts of crazy lines very fast and with little thought on the analysis board.
Any comments would be much appreciated!