When you see a player is just calling (limping) into a pot you need to be suspicious.
This is especially true if they are usually an aggressive bettor.
Sometime they are on a draw but they may also have the nuts and just be letting you do the betting for them.
If they were in the Big blind they may have gotten a big blind special so pay attention and take a little extra time to try and figure out what they would be calling you with. Don’t keep betting into them blindly.

When the flop is being dealt you should be looking at the other players faces and body language not at the flop. See if they make a facial expression that indicates strength or weakness or notice if they move toward grabbing their chips . If they reach towards chips they likely have a hand. Additionally, players who look away from the flop generally have a strong hand. They want to seem disinterested so they look away. Players that stare at the flop often have nothing. Remember this is a game about reading people and you need to pay attention to the non-verbal cues if you want to be a winner.

There are numerous reasons that can make a seat or table change a good move. If you have a very aggressive player to your left who is raising every hand and you never get to see a flop it may be time for a switch. If someone is abusive or too drunk you may want to get up too. If you crave action and you are playing with a very tight crew move to a more aggressive table. It is your right to move and you should do if you feel the need. A word of caution. In some instance in a Casino card room if the tables are too short they may not let you move as they want to keep the players per table at an equal level. Casinos and players don’t want 5 people at one table and 9 at another but it certainly cannot hurt to ask.

Yes they may be light in chips but they have nothing to lose so it is harder to tell if they are making a move with rags or the nuts. If you have a big chip stack you may want to try to take them down but if your stack is marginal and there are several players left to act don’t try to be the hero unless you have the right cards or pot odds especially if you are close to making it into the money.

You have to pay attention to what is going on at the table if you want to make it deep into a tourney.
When out of the hand spend your time looking at the players and their actions rather than texting and tweeting. You can pick up a lot of information by observing the betting, tells and other behavior. You can be sure that the serious players are watching you.

If you are playing in a low level cash game like a $1-$2 no limit many casinos have a max buy-in. However if someone has been winning at the table for hours they may have a significantly higher numer of chips than the max buy. Many people are afraid to come in with a short stack say $100 when many of the players at the table have 5x’s that amount. While it is true that in some ways you are at a disadvantage with the short stack, if you are disciplined and getting cards it can be a good opportunity to win a lot of chips with a smaller downside. This is not the time to do a lot of bluffing nor is it suggested that you call smalll bets and whittle away your stack. You are best off waiting for good cards and then making bold moves. If you are not getting hands you really have to be folding but if the cards are coming your way with a lot of chips on the table you have a good chance of having a nice win even starting with the small stack

In a heads up game, if someone bets and you call, you paid to see their cards and they have to show them to you. You do not have to show your hand if you don’t beat them and typically should not as it gives the other players information.
Your competitor can request that you show but doing so on a frequent basis is not encouraged. In fact some card rooms do not permit it.
In a multi-handed showdown usually the last player to take action by either betting or raising is the first to show the hand. In the case where everyone checks (or is all-in) on the last round of betting, then the first player to the left of the dealer button is the first to show their hand.

Some people engage you in conversation to be friendly but many do it to see if they can pick up a tell. Just saw it happen with Antonio Esfandiari and Vanessa Selbst on High Stakes Poker. Don’t talk during the hand. You may be giving something away and not even know it.

A student was telling me that she had a long way to go before perfecting her poker face and was riddled with tells. One thing you can start to practice right away is counting to 10 or 20 before you make any move whether it be bet, raise or fold. The consistency of action time will make it harder for people to read you.
Staring at one spot on the wall can also make you harder to read. Try to refrain from engaging in chit chat while in the hand. Your voice can give away a lot! A good pair of sunglesses can help also.

Sometimes after a showdown your opponent will declare that they have a better hand then they actual have to get you to muck your cards and then when they turn their cards over “pretend: to have misread their hand.
Make sure you see their cards before you muck.
It is a growing form of cheating.