When I wanted to hike it a few years ago, I was also unsuccessful in the "day before" lottery. From what I'd read, there was a pretty good chance of hiking up to the ranger checkpoint at the base of Half Dome and asking parties if there were any unused spots on their permits due to late cancellations or no shows. I let the ranger know what I was doing and sat down near by. I think I waited maybe 10 minutes before I got lucky when a small group of guys came up and said yes to my inquiry. Once the ranger had checked their permit, I asked her if I was good to go and she gave me the green light. No shows on permits happen all the time (as with walk up permits for Whitney) and I wouldn't hesitate to try the same strategy again as long as it's for just one or two people.
