Over the long term rewards become expensive and can keep you focused on checking off tasks rather than engaging with them, but they are still a useful motivational tool. They can be a great short term tactic to temporarily boost your productivity, and a burst of progress can really shake you out of a slump, especially if you tear through a bunch of mundane tasks that have been clogging up your to-do list for weeks.
You can just set yourself a target and pick a reward, but I have a system that I find especially effective. I use a dice and hand tally. The rules are very simple. I roll the dice each time I complete a task and click the hand tally if I get a six. When the tally reaches ten, I reward myself.
This is effective for a few reasons. The mild gambling element keeps things interesting but means that rewards are less frequent, so the prizes can be bigger and more compelling. Psychologically rolling to reach the ten count also seems far closer than a straight-forward race to sixty.
The tactile nature of the process is good for behavioural reinforcement – clicking the tally feels satisfying. The objects themselves also serve as a physical reminder to keep me focused.