Don't underestimate the power of praise as a self-reward. It feels really goofy to explicitly praise myself -- especially to do it out loud -- but that doesn't mean it doesn't work.

IME, the biggest problem with self-reward, whatever the mechanism, is that it requires quite a lot of discipline to differentially reward the thing I want to reinforce at all consistently.

The only time I ever really maintained that discipline for any length of time was when I was recovering from brain damage, when continued focus on self-improvement was the single most important thing in my life for about 18 months. In my real life, I just don't care that much. YMMV.

Recruiting allies to reward me works better for me.