I use Anki.
It's not for everyone, but I have, among other types, cards asking me to prove things, especially important results. I think this is nice because it forces me to remember definitions and the gist of proof tricks and techniques as well as being able to make it all work. But ain't it mere memorization? To some degree, yes, inevitably so, but the best way to remember highly structured information such as mathematics is to understand it really well.
I also find the first couple of months I am repping on new cards to be invaluable in helping me grok the ideas and contextualize them. I do very few problems and exercises but when I do tackle them, I am usually able to perform well just from this practice.
This method would be particularly well-suited for fields with lots of details and neat proofs that connect well with other things I learned and that I go deeply into. It sucks when I am dealing with unintuitive and long proofs or with fields in which I am only dipping my toes. Sometimes I break long proofs down. Also, there still is stuff I don't remember well and this gets worse the longer since I have used or studied them.
In my experience though, my retention with Anki is night and day better and I enjoy studying with it. First, it is highly structured so there is little guesswork and I trust the system. Second, it forces me to be disciplined and helps me focus.