There are many pairs of words that give me trouble. Here are two of them: farther, further and loose, lose.
Today when I was writing I looked these words up, and tried to pound the usage differences into my memory capsule. Farther is how long you're going to have to keep walking to get to where you need to be. It's always about the physical distance you have to travel. Traditionally, further was referring to a metaphorical distance, such as, "To believe his promises, I had to go out further on a limb than I had ever been before." However, the uses of further and farther have been changing. While farther continues to always be the only choice permitted by the grammar police when a measurable material length is the subject, further and farther are now correctly used interchangeably in all other instances. "How much further am I going to go on talking about this?" is just as proper to say as, "How much farther do we have to put up with these shenanigans?" Who knew?
Lose and loose, on the other hand, have very definite rules about which to use when. I just have a hard time remembering. Loose is an adjective, referring to a noun: Briefs are too loose so they fall off, the horse is loose because you forgot to shut the door, etc. Lose is to forget where you put the darned thing. It's a verb, an action you take. A frequent trick mentioned is if you let something loose (with 2 o's), you lose it (lost an o). Does that help? That tip doesn't really work for me. Ok, how about if I made it get lost (4 letters), I lose it (4 letters). Hmm. No, Ok, try again. Loose describes the state of some thing, and lose is something I do. The difference is hard to describe, but the more times I read the rules and talk about them, the more the distinction sticks in my brain.