Similar to having a Productivity Accountability Partner (tool 1.13), knowing you have to report to someone about your exercise or diet progress may make you more likely to follow through.
With exercise, one way to do this is by agreeing to exercise with a friend—that way you can both be accountability buddies for each other as neither will want to disappoint the other by not showing up to work out. Having a coach assign workout routines and goals and then expect you to check in periodically is another method to apply this tool. With dieting, belonging to a group where you report weigh-ins can be helpful, as can logging your food entries into a journal that you are expected to share with others.
Otherwise, your doctor could be an accountability buddy. You may not need to report in with your exercise or diet on a regular basis, but when you get follow up lab work that shows your cholesterol levels or blood sugar levels, you doctor will know whether you’ve been sticking to a plan.
If you don’t feel comfortable with, or just don’t want to have, a someone you know well be an accountability buddy, there are systems or apps you can download that will do this for you. In these you can compete against other people (competitors makes great accountability buddies) or create fake accountability partners and compete against yourself.