The brain responds most to random intermittent reinforcement. This is how slot machines operate. So a simple protocol is to reward yourself at random intervals for goal completion. So say you have a goal of working out five days a week and four of the days you do it. On each of those four days flip a coin and if you get heads, you have your protein shake or whatever reward, and if it’s tails you do nothing.

REF — Dr. Andrew Huberman goals toolkit