A good way to retain mathematical understanding?

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.

Committing things to the long term memory is not easy. There is not generally accepted magic trick that will make you remember something forever. From what I understand the things that we do end up storing in our long term memory are what we have made a routine and have done for a long time. (There are exceptions such as traumatic events that you can't remove from your long term memory)

If you have memories in your long term memory you can try in some way to associate the concept that you memorize to that memory. For example, if you remember the way your childhood room looked like, then you can associate each of the things to parts of a definition. Doing this, will make it a lot easier to remember definitions and is what many people do when they compete in competitions about remembering.

I know that some teachers recommend doing something similar to the above in that they recommend trying to visualize the concept. Can you make up a mental image of the definition? Can you find an image of what a normal subgroup is? (kernel of homomorphism maybe ...)

Many researchers don't actually try to remember the exact definition of something, but they remember the concepts by knowing the deep meaning. That means that you should ask yourself why is the definition as it is? Definitions in mathematics are not arbitrary, they have a motivation. Figuring out the motivation behind a definition can, however, be difficult. And so you might want to ask your professors about some of all of this.

I think, though, that best way is to make it routine. You have to do it a lot. Reading something one time will not put it in the long term memory. I also want to echo what others have already said: Teach. If you can find a way to teach the concepts some someone else, then that will definitely help both with the understanding and the memorization.