A good way to retain mathematical understanding? What is a good way to remember math concepts/definitions and commit them to long term memory?

Background:

In my current situation, I'm at an undergraduate institution where I have to take a lot of core classes and easy math classes because there are no higher level courses available. Therefore, I have to study math on my own. It worked out fairly well, but I have a big problem when there's a lapse in my studying. For example, in February, I was working on transfer college applications, so I was unable to study math. Right now, I have less than a month left in school, so I need to work on all my term papers. It seems that when I take these extended breaks from studying math, like 2+ weeks, I have trouble going back, and occasionally I have to re learn a lot of the material. Previously, I was able to understand the definitions, theorems, etc and solve some of the exercises. Now, I need to look back on the definitions to understand what I wrote down! It's frustrating that I'm regressing in my mathematical education when I'm not actually studying math.
Teach.

Seriously it is absolutely the number one way to learn.
Take a look at http://www.supermemo.com/

This system (and the method it describes) is based on repetition over a changing interval of time - short time intervals first, then each interval longer than the preceding interval. The author is rigorous in his approach to analyzing how memory is reinforced.
What is repeated is putting items INTO memory, and and TAKING THEM OUT (recalling them). Both actions are components of memory, that is, being able to "memorize" and being able to "recall" from memory.
The continual use of this technique also results in a second-order effect: an improved ability to memorize things in a shorter time, and with fewer repetitions.

DISCLAIMER: I have no relationship to the author of this method, nor do I know the author personally. For me this was just a useful discovery.