Hang around with at least a couple of instructors. All teach a little different and you will learn alot more if you DM for 4 or 5 rather than just one.
Hang round the Service shop of your LDS you can learn an heap from the service technicians.
Skills Skills Skills. go down to your local dive or pool and practice them over and over to presentation quality. Take an instructor with you to watch you doing them so he can make comment.
Do your skills slowly and break them down into small steps.
Listen to what your instructors have to say, and take note of how and what they like there DM to do.
Don't answer questions of students if you are not 120% sure of the correct answer, instead direct them to the instructor or go find the answer out yourself first.
Never contradict the instructor in front of a class, wait till you are alone if you have questions of him.
And Finally welcome to the world of tank carrying, always carry two, one in each hand to avoid back problems later in life. When carrying equipment bend from the knees, you will be carrying heaps.
Do not expect to get paid for DM'ing, you do it for the love of carrying other peoples equipment, filling tanks and working your butt off.
If you have any specific questions let me know off forum and I will happily answer them for you
Cheers
Chris
PADI MSDT
http://homepage.mac.com/sydney_diver