It seems like when people think about tells the only thing that all they are thinking about is a player giving up the strength of their hand. There is other information that players should be looking for and putting to use. Some may think that it's best to look at your cards early while no one is paying attention to them. While I don't disagree that the time a player is most likely to give away information about the strength of their hands is when they first look at it, I also feel like this is the time when players will be most guarded. By no means is this the only opportunity to gather useful information. While players may be careful to not to react when looking at their cards, once they are done looking they may relax and let their guard down.
The thing about a tell is that players are usually unconscious of them. If it was something that they were conscious of doing, then they would probably just not do it. They may give something away in their body language, or the placement of their cards or any number of small gestures. It might be something like a player capping his cards when he intends to play, but not doing so when he sees a hand he intends to fold. I might notice that the guy who has been constantly shuffling his chips, suddenly stopped after checking his cards. Sometimes I notice a player suddenly goes still and fixes their eyes on a one spot as if trying not to give something away, which tells me there is something to give away. And that is enough to start with. Sometimes a player may look very relaxed, but give the distinct impression that you would have to cut a finger off to get them to take their hand off their cards. Players who are worried about keeping the game moving are often so concerned about expediting the game that they position their cards so that they have a straight shot back to the dealer as soon as action reaches them. The guy who's sitting straight up, leaning slightly forward intent on the action, that just finished eyeing everyone else at the table's stacks has probably got a hand. The guy leaning back in his chair that seems more concerned about the location of a waitress than the action at the table, not so much.
One thing that I always take note of is which players habitually look at their cards as soon as they are dealt. When action is on me, the last thing I do before I look at my own cards is to look around the table at those players to see if I can pick up any information. Of course I'm not going to be able to put players on exact hands, or even know for sure how strong each player's hand is. How many players can do that? But I might see something that tells me that a player is likely to play their hand. I know all eyes will be on me when I look, but as I said, this is the time when most players will be most guarded. So, if I can manage to maintain even a little bit of composure I'm probably going to be okay.
And here's something else that people seem to overlook. We are all going to give away some information about our hands. When action is on us, we are going to have to act. If we don't like our hand everyone will know when we fold. If we do like it, they will know when we reach for chips. The thing is not to give that information away early. You don't want me to be UTG +1 looking across the table knowing I have one fold in front of me, two players who's body language indicates that they are extremely likely to play their hands, two others who's body language tells me that they are likely to fold, and a player in the BB who likes to defend. This is information I can use to determine whether or not I will play the cards that have been dealt to me, whether or not I raise, and if I do how much. I might decide that the hand is playable but, not worth the raise that I would have to make to try to get it around the table and past the two players that seem really intent on playing without too many callers. If one or both of them call then the BB will probably feel priced in to call with anything halfway decent. Not to mention that there are three players that I have no idea what they might do, or what kind of hands they might have. One of them just isn't giving anything away. There isn't anything for the other two to give away, because they haven't looked at their hands yet. Again, all eyes will be on them when they look. So if they do anything that indicates whether or not they like their hand or intend to play it, like folding or reaching for chips, everyone will instantly know. But since action is going to be on them, that cat is about to come out of the bag anyway. But by then it will be too late to affect my decision, because I will have already acted. That's the nature of the game. The players that act later will have more information than the players that act early, that is unless they do something to that gives that information away ahead of time.
Do what works for you. Play the way you play. As for me, I prefer not to look at my cards until it's my turn to act. I feel fairly confident about my ability to look at my cards and call, raise or fold without giving any other information away. I'm not as confident in my ability maintain 100% discipline at all times without ever slipping. It would be arrogant of me to think that I could. But I do know with absolute certainty that I can't give away information that I don't have. I don't know why that doesn't make sense to more people.