Things a substitute should not do:
1. Use how many years you've been a teacher as a threat or reason why kids should listen to you. Kids don't care how many years you've been a teacher. In fact, it's more fun to terrorize a seasoned teacher who in all her "30 years of teaching has never seen kids so noisy" (which I'm sure is also not at all true, she has definitely seen kids that noisy). What class wouldn't want to be the ones to set the record for noisiest kids in 30 years?
2. Ask the kids what their classroom routine is and then go with your own thing anyway. Kids need routine. In fact, the teachers want them to do the routine. It's part of their education--to learn how to do routines. And kids like routine, as much as they may not think they do. Besides, you don't want to end up having to deal with the kids who can't handle change in routine. But if you're really stuck on doing your own thing, then just do it. Don't dangle the classroom routine in front of the kids and then throw it away.
3. Wear a fanny pack to keep your markers in. Putting on the fanny pack is your first mistake. If you want to bring markers, keep them in your bag. And there are often several markers already by the board. You can use those. There's no need to keep your own personal set of markers out and use them possessively (while making the kids use only the markers on the board since the ones in your hand/fanny pack are only yours). Also, really, ditch the fanny pack.
4. Yell excessively at the kids/be too strict. Lighten up. You're a substitute, for crying out loud. The kids will only be worse if you make them dislike you.
What am I forgetting?