If you’re looking for the best advice on grammar and word usage, I recommend you check out the Grammar Girl. But what I can offer you are my silly tricks for remembering when to use reign or rein.
Rein applies to horses…or, perhaps, to other sled-pulling animals: reindeer.
Reign includes a “g” as does the word king. Reign is also found in the word “sovereign.”
I do highly recommend the Grammar Girl’s website and podcast. Unlike me, she tackles all grammar problems with confidence and aplomb. (I, on the other hand, will abandon any problem that cannot yield to a silly trick. Affect vs. effect? I won’t use either of them!)