Most people don't know, but microphone placement is one of the single most important components of your communication system. At PCI, we use noise canceling microphones that are specially designed to pick up your voice - not your engine, wind or shock noise. For the microphone to best determine what noise is what, the mic literally needs to be touching your lips. If you are having a hard time hearing anyone over the radio, ask them "how far away from your lips is that microphone?" If their answer isn't, "It's touching my lips!" then it is too far. When a co-driver has a microphone that is too far from their mouth, drivers will often turn up their volume, introducing other noise into the intercom system.
Always have the microphone centered left to right and top to bottom directly in front of your mouth and project your voice directly at the microphone. This is even more important for soft spoken people and children. DON'T YELL! You should never have to yell into your microphone, in fact yelling or excitement often makes you even harder to understand. Try to remain calm (which can be tough in a race environment), relax, take a deep breath, know what you want to say before you begin speaking, and clearly talk out, directly into the microphone.
These tips apply to anyone with a helmet or headset, get some mic socks and get comfy with your microphone - it will make the person on the other end of the radio a much happier person!