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.

I find that this site is very helpful for retaining understanding. A short visit to this site a couple of times per week is helpful, I think.