Ant if you have trouble balancing you should practice balancing off the track.
In fact do it between motos by pulling your bike up to a car tire and trying to balance on flat surface. Its harder to balance on flat ground do this every race and between motos. In just a few weeks you will balance like a pro.
Also practice balancing sitting down. You can never get a solid start until your balance is solid. Once you can balance for like ever without touching down your good to go.
To further help your start do some lil things.
Flex your fingers on the grips to help you relax. Alot of riders choke the bars the to death. Relax. Also take a deep breath on the gate and breathe out.
Remind yourself its just a race and it aint no big deal.
Finally your ready to slam the gate down. Dont wait for it to fall, make sure you move your hips forward and up before the gate drops.
If your constantly then clipping the gate you can make one of two adjustments or a combination of both. 1. leave a split second later and/or 2. lower your forward pedal one click. If still clipping try lowering it another click.
One other area you can adjust is your body position. But again never make extreme changes. Your body should be centered over your bike/seat. You will notice some riders that lean way back over the rear tire and still get a great start. Well they developed a good start from a bad habit. Dont copy, center your body.
Practice makes perfect. Even if you cant race but can make it to the track do some gates.
Be careful not to wear yourself out on race day doing gates.