The most common mistake is starting too hard only to be left gasping for air after a couple of minutes. Remember that starts are always difficult. Start aggressively and expect to suffer a bit (this is why the warm-up is so important). But, try to start at a pace that's manageable for your legs and lungs. You'll soon settle into a good rhythm and feel like you can push a bit harder. In a road race, you should try to stay with the pack. Racing off road, you'll probably have chances to pass people. But ride steady and within your limits. Going super hard to get ahead of a few more people might cost you later in the race.