*Update: There's a similar more in-depth post here called What Not To Do When You Teach English.
Here's a list of common mistakes new ESL teachers frequently make. I have seen other teachers make these at schools or read of them online in forums. I made most of these mistakes when I first started teaching English too.
Lesson planning mistakes
I'll just show up to class today and wing it. I am good at being spontaneous, I'll just make it up as I go along.
This often leads to a lesson with a lot of lows and wasted time. You won't feel very comfortable if you find yourself scratching your head as you think about what to do next with a classroom of students staring at you. Preparing can be as easy as a simple outline written down. Learn how to lesson plan.
Same old games and activities
I don't know what activity to use, so I'll just use the same one as before.
Your students will get bored if you always use the same games and activities. Games and activities are supposed to break up the repetitiveness of language learning not add to it. You need a diversity of tools, games and activities.
Not preparing the tape or CD
I don't have time now to set the tape or CD, so I'll just figure it out later in class.
This can be annoying and a lot of time can be spent fiddling with the tape or CD to find the proper spot. Make sure that you have the right tape or CD and that you set it up before class starts. This will make for a better class and you won't lose momentum.
Classroom management mistakes
Playing "nice guy" or "cool teacher"
Well, maybe I'll just be really nice to them or I'll be the cool teacher. I think that if everyone likes me or thinks that I am cool then they'll do what I say.
Wrong. This will often lead to disrespect. Don't play the "nice guy" or the "cool teacher". This doesn't mean to imply that you should be mean or unkind. But just focus on getting their respect first.
Maybe if I give them candy they'll do what I say.
No. Are you trying to give the child a cavity or a sugar high? Candy is like a drug to kids and if you give it to them one time then they'll likely ask you to give it to them again. Baiting kids with candy or anything else is like priming them for the rat race. Here's a do not do example.
For kids: If you do a good job (or stop messing around, be quiet, get an "A", etc.) I'll give you some candy.
For adults: If you come in on time, work hard from 9-5 and do what I say, I'll give you a bonus at the end of the year or maybe just a check at the end of the month.
What's the difference? I think you'll see that there is not much of a difference between the two. Candy is just one example, but there are other kinds of...
I think they will do what I tell them if I give them candy, stickers, stamps or any other kind of bait.
Wrong move. Essentially the reward must be learning. If used extrinsic rewards should be seldom and spontaneous. Otherwise learning becomes a means to an end and they'll loose interest in the learning and become focused on the reward.
Maybe they will quiet down if I just yell a little louder. Or maybe they will behave better if I yell louder.
You might intimidate a few, but you're just as likely to see a few students grinning back at you. The effects of yelling are just temporary and it won't win you any more respect. Without another means of solving the problem your students will likely revert to whatever caused you to yell in the first place.