I'm still looking for my post on concrete starts but untill I find it, this ought to keep you busy.
Nevermind me looking for it. Concrete starts are not that much different than dirt. The key is to gain the most traction and get off the gate the fastest anyway so here goes.
Clean the start pad the best you can. When you get on it (bike) spin the tire just a bit to get as much dirt off the wheel as you can. Don't do a burn out, just clean the tire, a turn or two should do the trick. Here's the rest.
What I learnt from the "Dr. D" school of starts.
Take the time to prep you're starting area. Make sure the gate falls smooth and pack the loose stuff at least 15' to 20' out from the start. If possible take a gate where you are not next to a rider and lining up perfectly in the center of the gate may not be the best place to be. Get there early enough to watch how the gate person operates the gate, usually there is a pattern.
When it's time to line up at the gate, be the last one to pull all the way up but leave as much room as possible between you and the gate. Sometimes this means the person at the gate having to tell you to pull up farther.
Wait till the last minute to put on your goggles. Don't do burnout's as you're at the gate, it'll ruin all the prep work you just did. Just rock the bike back and forth a couple of times to help in packing the dirt. (Disregard if concrete)
In the final seconds before the start scoot up as far as you can on the tank, set your butt down and let the skin on your butt sort of roll back. When you take off it'll help keep your weight from an abrupt transfer and help control wheel stands. Don't look directly at the gate, watch it from your peripheral vision. Look where you want to go. Keep both feet down after making sure you are in 2nd gear and hold a constant 1/4 to 1/2 throttle. As you see the gate start to drop it's time to go WFO and control your traction with the clutch. Don't be too quick to bring your feet up, do it in a "flowing motion". The quicker you can get your bars in front of the person next to you the less of a chance you have of going down should you contact. 1st corner you should be last on the brakes and 1st on the gas, remember there is no coasting in racing, it's either full gas or full brakes. (I added that but everyone knows that would be the "perfect" race situation) (The real thing is knowing how much, when and where to do all the above.)
Oh ya, forgot to mention you should look for the shortest / straightest shot to the 1st turn. Sometimes it may be faster to take into consideration the 1st or even 2nd turn on your starts. Make sure you take a line choice that will enable you to make the smoothest / fastest transition to and through the next obstacle.
This concludes all I know about starts that I can put into words.