For OLLs: Some are inverses of each other. For example, Sune and Anti-Sune are inverses, so you can do one and then the other, allowing you to practice both. There are a number of other less obvious inverses, but I don't know full OLL, so I don't know them all; only the ones with all edges oriented.
For PLLs: Most PLLs are their own inverses. So do the R perm and it will set up the R perm case. I.E., do R twice from a solved cube and it will be solved again. Obviously the Aa and Ab are inverses, as are Ua and Ub. The Gs are two sets of inverses also. You can practice the inverses in pairs and thus get practice on both. For the Rs, T, V, E, etc, again you just do them twice and you'll get back a solved cube. Repetition will help a lot.
Breaking any alg (OLL or PLL) down into groups helps me a lot. The brackets or parenthesis you see in algs are designed to show you how the author groups that alg into smaller groups that are easier to remember.
Once you've got some (or all) PLLs memorized, then you'll want to drill on recognition, which is a whole different thing, and why I wrote some software to help with that.
Brian.