Pedaling technique is the thing that has transformed my sportive performance - from climbing kitten to climbing dog.
Smoothing out your pedal stroke and "pulling up" with the leg that isn't pressing down on the pedal can make a real difference to your climbing ability. The additional power you get from using this technique can get you up hills that previously defeated you, and preserve your legs during a long sportive so you are in good shape for the final hills.
Here's a video that describes and demonstrates the technique.
At first it can feel odd to "pull up" but after a while it becomes second nature. You seem to get an additional boost with no extra energy cost - and soon you are zipping up hills.