A big bong hit.....Just kidding...
You have to teach your mind and body to deal with high intensity effort such as a race start. It's equal measures mental and physical IMO. I suggest starting some rides with solid climb something that you suffer with for the first few minutes. Train yourself until you get to the point where you don't suffer those first few minutes anymore. Do some race simulations. I've been using Strava to help me push harder on uphill sprints and the data has helped me a lot.
A good warm-up will help as all have said and mandatory. Once I get into the start line, I do some breathing exercises. Specifically heat and energy creating "fire in belly" type breathing to sustain the heat and energy I created in the warm-up. A few "ohhmmms" on exhale are a good measure too. It's a great psych out to your competitors and you can still talk smack between breaths......
Silliness aside it helps keep the energy level (heat) as you wait for the start and helps quell the pre race jitters. The other trick on starts is to know your competition. I'm a back of the mid pack scrapper on a good day. So I set my expectations and make sure to pace my start accordingly knowing that those skinny fast guys will be ahead of me know matter what. Learning your race pace is key. There will be guys that blow up mid race, you can reel those guys in and finish strong.
The best training for racing is racing. So enjoy a few races with the mindset of a beginner and lower your expectations and you'll gain huge strides in your race craft.