This year I listed all of the ones that didn't have websites yet on sedo, and set all of them to a couple of hundred pounds each buy-it-now. The idea is that either I focus on building an idea quickly, or risk that someone who thinks he has a better idea than me snaps the domain name up in the meantime.
Hasn't worked. But, so far three of those domains have been sold, so that essentially covers all my domain name renewal fees this year.
Perhaps I should drop the prices to increase the pressure on myself.