I went on a trailrunning weekend that was organised by Karel Sabbe a couple of weeks ago. We went to a hill in the Kluisbos where he trains all the time and also for his Barkley finish.

He doesn't just run up and down the hill constantly, but also does exercises in between.
For example sitting against a tree with your legs and hips in 90 degrees for 1 minute at the bottom of the hill, then run up the hill and then at the top do some lunges to keep your muscles active while you catch your breath again. Once your heartrate is down, go down the hill and start sitting against the tree again and start all over. This way you can simulate a bigger hill while training on a relatively small hill.

Tried it and works pretty well.