Know whether you'll be dealing...
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.
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.
This is what you should be...
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.
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.