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!)
Kelly Horn 
I love Grammar Girl’s book! Haven’t listened to her podcasts yet (anything that involves downloading big files I have to stay away from until we get a good internet provider). I have no problems with rein/reign or affect/effect. I learned a new one recently that isn’t too hard to tell apart either: eruption vs. irruption. 🙂 We’ve had an irruption of certain birds this season.