The intervals and sprints mentioned by others sound good to me but here's a couple more ideas:
1. Individual leg training- spin for 30 seconds using left only and an easy gear... brace off leg on the trainer arm... recover using both legs for a minute then switch legs. Don't overdo this... it is hard. Increase time and sets as you get stronger.
2. Low cadence stands- shift to a pretty hard gear and stand for a minute at 50-60 cadence... then sit down and shift to a somewhat easier gear for one minute at normal brisk cadence as if the hill had gotten easier but you are still going uphill a bit. Recover 3-5 minutes as needed and repeat a few times as you are able.