If you aren't aware, my website has a bunch of crossword puzzles on it, written by various friends and family. In my editing of these puzzles, I've tried to adhere to an implicit language that I've noticed in other crossword puzzles that I've played. The page is an attempt to write down a bunch of observations and idioms I've noticed, as a curiosity and perhaps also a guide to folks contributing more crosswords.
While it is usually more enjoyable if a puzzle offers some challenge, it is cleverness and insight that really makes a good puzzle. When thinking of clues, you often want to choose the most unexpected and surprising clue for each answer, rather than whatever is most likely to stump the reader. One should also have taste in how the whole set of clues fits together, of course, since you can go overboard in picking bizarre clues; the point is just that your goal is not to prevent the reader from finding the solution, but rather to teach them something or perhaps make them laugh out loud, like when they hear a good joke.
For example, if the answer is plural ("picks"), the clue should also be plural ("guitar playing tools"), and if the answer is singular ("pick"), the cluse should also be singular ("guitar playing tool"). Similarly, if the answer is a verb ("pick") should also have verb-looking clue ("play the guitar"). There are lots of other linguistic features that should match, such as tense, person, mood, etc. It's not important to understand all of these - I generally just follow my gut.
At first it may feel like crossword clues are just free association with the answer, but in my experience they are almost always a definition of the answer. They aren't always the most obvious or complete definition, but they do describe one meaning of the answer. What you don't want to happen is for someone to get stumped, look at the solution, and then think, "How was I supposed to figure that out?" Instead, they should think, "Ohhh, duh!" or maybe, "Ahh, okay, I guess so; that was tricky." Not every clue is going to be a success for every person, but you should at least play fair and not make people read your mind.
Word play is a great thing to incorporate into your clues, but be aware of when it is a bit of a stretch. If so, you can at a question mark at the end to denote that it is a stretch. This signals to the reader that the connection is a stretch and that they should perhaps do this clue toward the end of the puzzle, since it is probably harder than the others. You shouldn't make all the clues be extremely tricky like that, but it's fun to have one occasionally.