Flexbar worked great for me, and a few other people I know. I had tennis elbow for a year before I discovered the flex bar, and then it was gone in 3 days. When they did a clinical study to compare the flex bar to conventional treatment, they cut is short because the results with the flex bar were so much better.
Basher said..
Sail with a changed grip - for example, make yourself use an underhand grip if you normally go overhand on the boom.
Another good one. I had tennis elbow on just one side. I later noticed that it's the same side I use an underhand grip on - I only use overhand grip on the other side. Now, when I start feeling my tendons acting up a bit while sailing, I switch to overhand grip, and don't really have problems anymore. Before I did this, I'd need to use the flexbar a few days every couple of months or so.