Some other tips on starting to remember:

• On concrete starts you can warm up your tires about 30 seconds prior to the gate dropping to wear the edges (of a new tire) and also warm the tires up for better traction.
• Practice makes perfect, practicing on different surfaces will fine tune your ability to adapt at different tracks.
• Often tracks will have a log placed behind the start area to keep riders from rolling the start, if the ground is not level it may cause your bike to roll forward, if you prefer not to hold your front brake, consider digging the dirt away and if thats not possible, consider building a small mound of dirt in front of your rear tire to keep your bike from rolling forward, this dirt will cause very little resistance if anything.
• Once I have packed the ground in my starting position I like to kick the ground with the front of my boot to give the ground small raised edges to hopefully increase traction.