1. Make a promise, then make another promise to pass a law making it illegal for you to break the first promise, unless you bring forward another law to overturn the law you passed originally.
2. Make a promise, then carve it into a massive great rock and put it in your back garden, to remind you of the promise you made.
3. Make a promise, then put it into a large document, and use red lines to mark out the promises you're really going to keep, as opposed to the ones that are neither here nor there. Note: get the right sort of red, infra-red is invisible. Mind you, you'll probably wish you'd used infra red after all.
4. Make a promise. Keep it.