Improve Your English
Do you know what that abbreviation means? I believe it comes from the military, but it can be applied to a lot of situations and certainly applies to ESL teaching.
Proper Planning Prevents Piss Poor Performance
Unless you're very lucky, lessons on the fly simply don't work, even if you've got "The Teacher's Book" you still need to take responsibility for the content and presentation of the lesson.
Make sure you have all the materials that you need in the classroom and that any equipment works and you know how to operate it. If you are using equipment, have a back up plan in case of problems. There's nothing worse than relying on an OHP, only to find that another teacher has "borrowed" it, or the bulb is on the blink.
Read through any pre prepared lessons and text first. Check it for errors (yes even text books contain mistakes) make a list of vocabulary to introduce, anticipate the questions that your students might ask. If there's going to be a discussion afterwards think up some interesting questions to stimulate discussions.
Make sure you have enough copies of any handouts and any books required.
Plan ahead as to the best way to get the students to interact: group work, pair work, drilling, individual study. Make sure the classroom is properly laid out for whatever you decide.
If you're going to play any games during the lesson, make sure you understand the rules. It's no good starting a game of vocabulary noughts and crosses, if you've forgotten how its played.
Have some props to hand. Print and cut out pictures from magazines or newspapers. Start a collection of items you can use to demonstrate different vocabulary. It's much more fun to practise shopping vocabulary when you have some play money, and or empty packets etc. to set up a little shop with. A clock is invaluable if you're practising telling the time. If your going to do clothing vocabulary having some cut out paper dolls and clothing to dress up is way more fun than just reading and talking about them.
After the lesson, with all the fun you've all had, make sure you leave the room as you found it.