With the following, there is "fun sharking" when you are playing friends for fun. But then there is nasty sharking such as in a tournament situation. I don't care about the fun sharking and I do it myself to friends (Usually very obvious sharking like standing in front of pocket and saying miss! miss! miss!).
I will not tolerate sharking during tournaments and I don't do it myself. If someone does shark me, I expect to win. Usually this is the only way they can win - by trying to make their opponent play worse. I on the other hand devote my energy to practice (not sharking), so I ususlly have the advantage.
Sharking can be very obvious. For example your opponent stands in front of the pocket you are aiming at and waves his arms in an attempt to distract you.
Or sharking can be quite subtle. You may be a favorite to win a tournament and about half way through the tournament, you have lost one match. Another favorite to win comes up to you and says "Have you won all your matches so far?" Well that guy knows darn well you lost a match. You saw him watching you play the match you lost...
If you go look at the player chart, you will see you play that guy next! So he is sharking you ahead of time. Trying to get you to think about losing rather than thinking about winning. The way to answer his question is to say "Yes, I have won all my matches so far!" If he says "I thought I saw you lose a match.", then say "If you know I lost a match, then why are you asking me?" Think about all the matches you have won to counter his shark.
Then before playing, your opponent may say "I've been watching you play. Is that a new grip you are using?" (He is trying to get you to consciously think about your grip while shooting which can throw you off.) When opponents I am about to play say stuff like this to me, I say "Are you sharking me?" They always deny everything, but clam up after that.
There is an out of print book on psychological sharking tactics. (Not specifically for pool.) It is "The Theory and Practice of Gamesmanship" by Potter. Interesting book.
Get used books here...
http://used.addall.com
In general, following are things people will do to shark...
Anger (Making your opponent angry)
*Wrongly accusing your opponent of cheating; accusations of improper lag, improper coin-flip, improper shot, etc.
*Accusing your opponent of something they did not do or did not intend to do.
Concentration (Disrupting your opponent's concentration)
*Doing anything vocal or making noise while your opponent shoots, is approaching the table to shoot, is studying the table for next shot, or is racking balls for next game. This includes talking to other people during the match.
*Any comments or questions about your opponents playing/shooting good or bad.
*Asking opponent why they are aiming at the wrong ball when they are aiming at the correct ball.
Distraction (If what you are doing would distract you while shooting, it probably distracts other players)
*Moving deliberately within the shooter's field of view.
*Standing where opponent needs to be for next shot.
*Waving your hand while your opponent is shooting or about to shoot.
*Standing at the table, especially behind the target pocket.
*Chalking your cue at table when it is your opponents turn to shoot.
Psychological Tactics (Upsetting your opponent or disrupting their confidence)
*Using psychological "tactics" before the match/tournament begins - Negative implications/comments/questions to players you may play in future games.
*Statements, implications, or questions about your opponents missed shots in prior games - Trying to make your opponent think about their poor shooting.
*Statements, implications, or questions about your opponent's losses in prior games - Trying to make your opponent think about the previous game they lost.
*Statements, implications, or questions about your opponent's exceptional playing abilities - Trying to make your opponent overconfident.
Unsolicited Instruction (Telling your opponent how to play better)
*Any statement about how to play better, what bridge to use, grip to use, etc.
*Coaching your opponent. Don't coach other players unless they ask for advice. Unsolicited coaching is a shark.
*Criticizing your opponent’s game.