It’s very simple. Every time you get a $5 bill, you aren’t allowed to spend it. Just save it! At the end of the year, you buy yourself something fantastic with your savings. Christmas shopping is another possibility.
You can use a large Ball canning jar or a shadow box. It would be nice to put your savings somewhere that you can see it as it grows. A visual reminder will be a good encourager to keep you going on your savings.