I mean, there's like 145,000 things to mention, but I suppose it depends on where you are as a player.
The biggest losing I see done from a stack is people making calls when they don't understand why they're calling.
You're playing a $1/$3 game, flop hands you middle pair and a gutshot, but there are two suited cards (or any number of other problematic boards). Two posistions in front of you a guy places a $25 dollar bet. "Okay", you think, "I'd pay $25 to see the next one with this guy". How about the people behind you? The guy directly on your left that might have flopped top two, or trips? What are you going to do when that guy makes it $65? If you're prepared to play for $65, why didn't you think of that first and place the raise yourself? Too expensive? You'd play for $25 but not $65. Why didn't you think about that before you called? Now you just dropped off another $25 from the stack.
Know when (cards, opponents or position) and Why to call, and when and why to raise or fold. Start asking yourself that at every street.
You also need to consider how you're going to handle the action going forward, depending on what the next card to drop is. You should be ready for multiple scenarios. E.G. "okay as long as I don't see a spade, a nine or a seven on the next street, I'm going to do "X" to try and win the pot." Or "if the board pairs I have to let this go against this guy, if not I can check it to him and make him think his two pair or trips are good".
Good luck my dude