Use spaced repetition software. This is the formal explanation about why it works. The intuitive explanation goes something like:

1. Spacing effect: the less frequently we review something, the stronger the memory.

2. We have computer programs that calculate when we forget things, so we only review them when we're about to forget.

3. This results in less frequent reviews. By (1), this results in a stronger memory, so the computer program calculates that your next review should be later, resulting in a stronger memory, resulting in an even later review...

The best program I'm aware of is anki.

Also, if you read through Gwern's article, you'll find this under External Links. May be helpful.