Rule #1: Don't be fickle. If you follow, then commit. Don't unfollow later. If you're fickle, you're also a fuckle. That's Scottish for fuckhole.*
Rule #2: Exercise due diligence before following. If you want to know whether someone is worth your time, do some research: read a couple hundred of that person's tweets. It takes only five minutes to read through that many tweets, and it's a good investment of your time. Does he have your political leanings? Does he match your sense of humor? Does he think the way you do? Does he provoke strong mental and emotional reactions because he thinks opposite the way you do? If yes, then go ahead and follow. If not, don't follow and then unfollow later. That's a stupid waste of time—your time and his.
Rule #3: Don't be random. Live your life as if rationality actually guides your decisions: you'll be a happier person for it. People who merely—and constantly—follow random impulses are fucking dumb. Have a reason for following someone other than, "Duh... he seems interesting..."—an assessment that you make after reading exactly two of that person's tweets.
Rule #4: Don't be needy. No one respects you when you're needy. Did you follow someone because you're just looking for a followback? Then you're a goddamn moron, and I'm better off without your stupid ass on my list. Fishing for followbacks is lame and a sign of insecurity. Stop it. Show some backbone. Have some pride in yourself.
By the same token, it's equally needy to do a followback out of a sense of polite reciprocity. You are under no obligation to follow anyone back. Remember that. You're not "returning a favor." Follow someone back because you aren't being fickle, because you've done your due diligence, and because you aren't being random or needy.
Twitter Neediness Quotient: [# following] / [# followers]
If the above fraction gives you a number above 1.0, you're a needy bastard, and that has to change right fucking now. You should always have more followers than people you're following. You're a human being, remember—not a hungry puppy trotting after people and hoping to receive some of their scraps. Have some dignity, for Christ's sake.
Rule #5: Block spam followers and barnacles. This rule is a bit different from the ones above it in that it applies to your "followers" list, but it's important to keep yourself honest about how many real people are actually following you. Maybe you think you're popular because your profile photo includes a hot chick showing off her cleavage. But how many of your followers are actual humans and not, say, bots and porn spammers?
Also: how many of your followers actually know you to some extent? How many have a real personal connection to you? How many have bothered to interact with you via an "@" message or a Twitter DM (i.e., direct message)? Some of the people who follow you are mere barnacles: they attach themselves to the hull of your Twitter account and just... hang there, doing nothing. Not reacting, not interacting—nothing. Again, what a goddamn waste of time. Life is short. Block the barnacles.
Rule #6: Don't follow if you say "true" to any of these statements:
•I'm following this person just because I want a followback.
•I'm following this person just because he followed me. As a courtesy, see.
•I have not read a couple hundred of this person's tweets before hitting "Follow."
•I'm unable to say what this person's sense of humor is like.
•I'm unable to say what this person's political views are like.
•I'm unable to say what this person's religious views are like.
•I have no idea whether this person thinks the way I do about any given topic.
If you said "yes" to any of the above, then don't follow. If you follow despite your ignorance, you're being an idiot. Again: life is short. Don't waste your precious time. If you want to follow someone, then be a wise follower. Choose carefully. Choose people whose tweets somehow enliven your day, make you laugh, make you go, "Huh... I never thought of that before." Be worthy by choosing the worthy. There are plenty of morons out there. Don't be one.
One last thought: in no way am I implying that you should follow only those whose views you agree with. Go ahead and follow people you disagree with: their points of view might piss you off, but they're guaranteed to make you think. And what could be better than associating with people who expand your mind?
*Regarding the fickle/fuckle/fuckhole etymology: I just pulled that out of my ass, so please don't take it seriously.
More to the point: I can think of only two legitimate reasons to unfollow after following: (1) the person you're unfollowing unfollowed you first, and you're honestly disappointed in that action; or (2) the person you're unfollowing hasn't tweeted anything worthwhile in months—he's either inactive or he spends all his time either retweeting stupid shit or linking to stupid shit, i.e., not tweeting his own original thoughts.
Hey, everyone's entitled to retweeting and linking now and then—even to the occasional retweeting/linking binge—but when a tweeter does nothing but retweet and link, you can be sure that that person has no original thoughts of his own. As Ben Kenobi once observed, "Who's the more foolish—the fool, or the fool who follows him?"
Don't be the bigger fool.