Ride with faster people. Beyond that, it depends on what your climbing weaknesses are. For example, for me, my biggest weaknesses are (1) a weak mental game, (2) poor endurance out of the saddle, and (3) poor leg strength seated, in that order.
All the pedaling in the world won't help (1). I'm working on pacing strategies to allow myself to better approach my physical limits. (That I never feel the need to throw up while climbing Baldy or Palomar should tell me something.) I've worked out new flat ground/shallow grade pacing strategies. Still haven't learned to do the same on steeper stuff. (2) and (3) are easier to target in my case. When I don't feel like riding outdoor (too lazy), I try to spend some time on a trainer at high resistance, in and out of the saddle. If I were more disciplined, I'd also hit the gym. But I'm not that disciplined, so some time on the trainer while watching TV or surfing the web will have to do.
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