If the class is large (sounds like it is) see if you can break them into two groups, no larger than six per instructor. Give your group two "stations" at a time; stay at one, and be sure you are very close to and can easily see the other station. Kids that age developmentally cannot remember more than one or two things at a time. If you have a larger obstacle course, try to focus on one main skill and think of the other points along the course as a way to get around and back to you without getting lost. Not that what you have them do along the way isn't important--walking across a floor beam, bear walking through cones, or tuck jumping into a series of hula hoops for example--but it should be something that doesn't require remembering a whole bunch of little points and corrections and skills and directions (a large, empty section of floor along the way that you may use to practice handstands with older kids is now an invitation to wander to those super cool balance beams two feet over). Always station yourself to where you can see every child easily.
When you are talking to the kids, try to get down to their level. Literally. Kneel down, look them in the eyes. Watch your voice level, and when possible, speak softly. The best way to keep a group under control is to gain their trust...they'll want to please you and you'll have more luck with other strategies you implement.
Rhyme. Use songs. Add sound effects. On the tramp, "freeze, bend your knees" is a good way to help the kids remember how to stop. Someone mentioned using "1 2 3, eyes on me!" to get their attention. You can add onto that--have them respond back, "1, 2, eyes on you!" and have them point to you. Or try a clapping rhythm: x x xxx (ok, that's difficult to explain in writing!!) You do it, then they repeat it. Or "if you can hear me, touch your nose. If you can hear me touch your head. If you can hear me, cover your mouth" or some version of that. All these things can help you get a groups attention without ever having to raise your voice.
Praise, praise, praise. Kids inately want to please you. If a child is misbehaving, point out children who are doing good things. "I like the way Susie is stretching like a big girl. I like how Jeremy is using good listening ears. Look how well Anna is standing in line for her turn!" Typically, the misbehaver will fall into place quickly--(s)he is not getting attention and wants yours now.
I'll keep thinking. I know I have some great articles around somewhere too that I'll look for.