This is why many pros wear sunglasses or visors/caps when playing, they know that the eyes rarely lie. For eg. many players can't help but stare at big hole cards, so their length of time peeking is longer. Conversely, if a player is looking to steal the pot, he may look to his left to see if the remaining players, who haven't yet acted, have quickly glanced at their cards and are likely to fold. Another eg., a player may try to ask you questions about your hand, knowing that people rarely can "look someone straight in the eyes" while being dishonest.

Facial Expression

Again, many pros try to disguise their entire face by wearing a cap and looking downward. This is to avoid the classic stare down that poker pros are famous for. They may try to study your face for nervousness (detecting a weak hand), or even look for repetitive characteristics like a body "tic". You may have obvious unhappiness in your face when your hand is weak, and conversely, you may show a contrasting show of confidence when your hand is strong.

Weak is Strong/Strong is Weak

In its most simplistic form, this usually applies to novice players but frequently applies to pros as well. Basically, players like to be actors, and when they have a monster hand they tend to look disinterested. eg. "Oh, is it my turn to act" or "Oh I guess I will play these cards". Alternatively, a player increasing the level of his voice while raising the pot, and trying to look intimidating. may be running a bluff.

Anxiety

Anxiety typically occurs in people when they are confronted, or anticipating confrontation. Psychologists call this the "Fight or Flight" stimulus response, which links back to the days when we were cavemen/cave women. Physical changes happen including flexing of muscles, eye pupil dilation, palpitating heart rate, dry throat. In poker, when someone has a big hand they are typically ready for confrontation and can exhibit some of these characteristics. You may see the chest expanding abnormally, or you may notice the players voice become slightly higher as he makes a comment. Some of the top players in the game will stare at the vein on the top-side of your face for blood pressure changes. During a bluff, the player may demonstrate anxiety, but if he knows he will fold the hand if re-raised (non-confrontational end) may look quite comfortable.

Trembling Hands

Also, a byproduct of anxiety, beware of a player whose hands are shaking, this nervousness can represent a big hand.

Glance at Chips

Again, relating to the eyes. It is common for players to quickly glance at their chips if they connect with the board after a Flop for example. This may be a subconscious reaction, but the player is already planning his attack.

Peeking Hole Cards on Flop

Some players will take another look at their hole cards when, for example, the board is showing a potential 3 card flush draw. Typically, the player is looking to see if one of his cards is connecting, because he remembers only that the two cards are different suits.

Repetitive Betting Patterns

Usually the most revealing tells are based on the way a player habitually bets during particular situations. For example, maybe the player always checks when he has made the nuts, or a player may regularly fold after being re-raised.

Body Posture/Attitude

Some players show obvious changes in their posture based on their attitude/hand strength. For instance, you may notice their shoulders drop/slump when they are not confident (weak hand). Conversely, you may notice the player is very attentive and sitting in an erect position when he has strength. You may also notice that a player who bluffs a lot, leans forward in a confrontational way... remember Strong is Weak!

Chip Stacking

When you first sit down at a table, study the way the players stack their chips. Although it is a generalization, loose aggressive players typically maintain unorganized/sloppy stacks, while tight conservative players keep well organized/neat stacks.