Everyone has leaks in their game, some are more extreme than others. If you take some time to identify the things you do at a poker table that cost you money you'll be adding to your overall expectation. Each leak you plug means more money (or less losses) over your poker playing career.
Playing Too Many Hands
This is the classic hold 'em leak, especially in brick and mortar cardrooms where it takes a more significant amount of time to deal each hand and it can seem like (and occasionally is!) hours between each playable hand. It can be very temping to play with borderline and sub-borderline hands.
In low limit games this manifests itself most often as playing any hand that contains an ace and any two suited cards. Both of these hands can cost you a lot of money.
Playing Hands Out Of Position
Along the lines of the previous example is the play of hands out of position. One common mistake is playing far too many hands in early position. If you find yourself calling bets with small suited connectors, or weak aces kings queens and jacks then you have a serious leak in your game. You should be avoiding the first few positions in general and when you do play them you should have something very strong to offset the positional disadvantage.
Save the more marginal hands for later position. In a low limit game you can always play 87 offsuit on the button for one bet if you have many people in the pot with you however you should *never* play it in early position.
Overcalling The River
Don't be the fourth person to call a bet on the river, or at least if you are you should probably have a hand strong enough to raise the river.
This is attempting to pull down too many pots through deception. Yes, sometimes you can take a pot on the flop if an ace falls and no one has an ace, however low limit hold 'em is a showdown game and in most cases you are going to have to show the best hand to win. If you find yourself bluff raising with K3 offsuit before the flop and then betting it the whole way then you can expect to lose a sizable pile of chips.
Bad Odds Chasing
The most common version of this is calling with bottom pair weak kicker in the hopes of making trips with your bottom pair or a weak two pair (a total of 5 outs). Although there are often enough people in the hand so that you could draw to this hand if it was the nuts, it will lose far too often even when you make it to be profitable.
Another common bad odds chase is you have a pocket pair. For example pocket queens vs. a flop of AKx. If there is action on the flop you are not only losing and drawing to a two-out possible win but whenever you make your hand you'll sometimes lose to someone with JT.
Yet people make this mistake all the time. Remember the low limit rule that if you don't flop a set or an overpair you are probably beaten and you should fold. Yes it'll hurt when your card comes on the river but it is a 10 to 1 shot with two cards to come and the pot will rarely justify those calls.
Tilt deserves an article for itself. Anyone who has played poker for any length of time has experienced tilt at one time or another during their life. Tilt happens for most people when they have a great hand that is killed, usually on the river, and often times by a horrible hand. The symptoms are severe cases of all the other leaks to the extreme. If you find yourself calling or raising with any two cards hoping to get away with impossible bluff and praying for miracle cards on the river then you, my friend, are on tilt.
Tilt is an emotional state and you need to be able to recognize it and control it. Sometimes the best control is to simply sit out for a few hands automatically when you take a bad beat if you know that you often go on tilt. Another way to combat tilt is to tighten up even more.
Playing At The Wrong Table
Ironically this leak is often overlooked and yet is probably one of the most costly leaks of all. When you play at a table where the players are too tough you are going to lose money. You'll often see even professionals make this mistake where they will sit down at a table full of world champions instead of at a table full of tourists. Don't make this critical mistake!
Getting Involved In Too Many Pots
Especially when there is a rake on the pot as there is online you do not want to get involved in a lot of pots. Specifically this means you're going to have to defend your blinds less often (however when you do defend them you'll want to make them pay extra before the flop) and it also means you're going be taking stabs at stealing the blinds less often.
The ideal pots you do want to get involved in are large multiway pots where you have a draw to the nuts to get your most bang for the buck vs. the rake that will be taken out of the pot.