I've updated our "learning equations" discussion over on the Tips page, so I wanted to share it here. Generally I avoid mnemonics when it comes to equations, but I've added an example of how I'd memorize 4/3 pi r^3 with memory techniques at the bottom.
How can I use memory techniques to learn equations?
Generally, I try to avoid "memorizing" equations. Again, the goal here is to learn efficiently by giving tangible meaning to what you're learning. In the case of equations, true understanding should be achievable, so memory techniques should generally take a backseat.
That said, I do use memory techniques for specific pieces of equations I find difficult to remember. So I'd recommend trying to identify the one or two tiny things that are tripping you up, and to encode those things specifically. For example, take this equation: