It's been 20 years since I was in a classroom. Thirteen years later I got into the online math help scene. But during that thirteen years, I kept myself active with math by formalizing my thoughts in a journal. I felt the best way to strengthen my understanding would be to investigate theorems and try to fluff them out as fully as I could, as if I was writing for a critical audience.
Then I got involved in math help forums, and eventually wound up here, where I found a site that matches my needs and has a philosophy of helping by engaging the student. I found naturally that helping others is the best way to help myself.
You will find that there are many folks here (many/most of which know a great deal more about mathematics than I do) who are quite happy and willing to enrich the understanding of others, particularly those who are motivated to learn.
So, my recommendation to anyone seeking to really learn material is to take given problems and generalize them as much as possible, and then investigate the behavior of the formulas at the boundaries. That's what I tried to do as a student, especially in physics. Once you can derive a formula from basic concepts, then you truly own it. ;)