Keep your body as far forward and as low as possible, keeping as much weight as you can on the front end to reduce the chances of a wheelie.
Avoid the temptation to watch other competitors for your cue. Concentrate on what you're doing, watching the flag or lights for the start, then minding your engine rpm as you leave the line.
Know whether you'll be dealing with a street tree or pro tree before you line up at the dragstrip. At a roadrace event, watch a few starts before your own if possible so you can see how the club works the lights or flag.
This is what you should be looking at or feeling for: keep your engine spinning at the launch rpm as you gradually open the throttle and release the clutch. If the rpm drops too low, the engine will bog and ruin your launch. Too much rpm and the front end will come up.
A good start is an essential part of a sprint race and practically the whole race at the dragstrip, making it important that racers at both venues master the art of a good launch. The trouble is that with just a handful of passes at a drag meet or a couple of sprints at a club event, practice is a bit hard to come by. There is a lot of action packed in just a couple of seconds of time, and whatever you can do to get to the strip regularly will pay off in better technique, for roadracers and dragracers alike.
A good launch begins well before the flag drops or the lights illuminate. The desired goal is to leave the line accelerating as hard as possible, and to do that you'll have to manage the throttle and clutch effectively to limit wheelspin and stop the front wheel from rising off the ground. Ideally, you want to deliver just enough power to the rear wheel so that your bike is on the verge of wheelspin or a wheelie, right from the nanosecond the light turns green. In your everyday riding you should become comfortable with slipping the clutch from a stop, and modulating both the clutch and the throttle as the bike accelerates. The more familiar you are with how your bike reacts under these conditions, the easier and more natural it will be for you to manipulate the controls properly without actually having to think about it.
To reduce the chances of a wheelie, position your body as low and as far forward as possible, keeping your upper body as low to the tank as you can. Note that wheelies have a snowball effect: the more the wheel comes up, the easier it is for it to come up even more. It's best to not let the wheel come up more than a couple of inches if at all, and the quicker you can get things under control the better.
Dragracers constantly refer to a "launch rpm", and you'll need to find the best engine speed for your bike to properly leave the line. Remember, you want just enough power to be on the verge of wheelspin or a wheelie; for a big bike such as a Hayabusa or ZX-14, you'll need just a few thousand rpm showing on the tachometer. A high-revving middleweight like an R6 will need 10,000 rpm or even more for a clean launch. Pick a launch rpm, and have your engine spinning at that rpm a couple of seconds before the start. Avoid blipping the throttle and racing the engine; keep a steady throttle hand and the revs constant at your chosen launch rpm.
At the dragstrip, a street tree illuminates the three yellow lights in 0.5-second intervals and the green another half-second later. A pro tree illuminates all the yellow lights at once, then the green 0.4 seconds after. A perfect human reaction time is about 0.4 seconds, so on a street tree you want to leave just after the final yellow light; on a pro tree, go when the three yellows illuminate. Don't watch for the green light, as you'll be too late; keep an eye on the bottom yellow light for your signal to go. Roadraces may begin with either a flag or light, and it's best to check out a few races before your own to see how your particular club's starter operates. For lights all you can do is watch the light itself, but on a flag start, watch the starter for a tell-tale such as the tip of the flag moving, the starter's elbow bending or the like.
Here is where practice through visualization comes in handy. From the sidelines, watch the starter or lights and think about the whole process; what you do beforehand, and what the first thing you do when the light changes or the flag drops. You can do this practically anytime, keying off stoplights on your way to work (although don't follow through with a racing start...), playing a dragracing video game or imagining the starter flagging the race off.
Once you've got the signal to go, you want to feed in the throttle and gradually release the clutch so that your bike stays at the launch rpm until road speed matches the engine speed. At that point, you should have the clutch out and be on the way to full throttle. Smaller bikes will require a lot of throttle and a lot of clutch slip, whereas on bigger bikes you will have the clutch out before you're at half throttle in general. Experienced riders will want to work toward using full throttle right from the start, even on a big bike, and using only the clutch to modulate the power delivery. If the front end comes up or the rear tire starts to spin, keep that in check by pulling the clutch in a tad rather than backing out of the throttle. Closing the throttle will drop the revs enough that it will be difficult to recover, whereas pulling the clutch in keeps the engine spinning hard for when you have things back under control. Many riders keep the throttle steady through the whole sequence until it's time to shift to second gear, but keep feeding in the throttle once the clutch is out until it's at the stop. On a big bike that still has lots of speed in first gear to go even after the clutch is out, there's lots of time to be made and you'll see this in the 120-foot times as opposed to the often-studied 60-foot times.
Once the pass or race is over, analyze what happened at the start and change your launch rpm accordingly in 1000-rpm increments. If you felt the engine bogged or you needed too much clutch slip, bump the launch rpm up a step. If you fought a wheelie or tire spin, lower the rpm a notch. You can also make adjustments by changing the rate at which you let the clutch out, or the rate you opened the throttle; the two are not tied together, and you should experiment with varying both the clutch and throttle. Before the next run, know exactly what you plan on doing and run the scenario through your mind a few times, so that you're well prepared.
Every bike reacts differently leaving the line, and it's up to you to find the best combination of rpm, clutch slip and throttle opening so that you launch as hard as possible right from the instant the light turns or the flag drops. Again, practice makes perfect and we can't stress enough how much a night at the dragstrip pays off for a roadracer, or the benefits of visualization for this particular technique. The thrill of getting the holeshot at your big race or running your first nine-second pass is well worth the effort.