Without knowing what kind of training/racing schedule you have it's difficult to specifically treat the problem but there are two things you could look at which might help.

1) training intensity/volume in relation to recovery time, and 2) Your post session recovery nutrition

1) It could be that you're simply not giving yourself enough downtime to recover from your training and races. Immunity is compromised by the amount and volume of training you do so make sure you take this seriously. Most riders fail to accept that highly intensive training sessions should be followed by sufficient recovery time, believing that if they are off the bike, their rivals are still out training and getting stronger. So look closely at how hard ou are training and plan sufficient recovery time to see maximum adaptations.

2) Make sure your post session/race recovery nutrition is rapid and sufficient for the effort you have just done. Use recovery shakes aggressively immediately on completing a session - a good protein-based recovery formula supplemented with L-Glutamine powder is a good place to start, and regular use of a Branch Chain Amino Acid supplement will also help.

On a more general note make sure your all-round diet is good, varied and plentiful, many cyclists fall into the habit of not eating enough as they're weight-watching, but careful monitoring of your diet/training relationship means you can eat lots without pilling on the pounds. If you have any doubts on what you should be eating, consult one of the many sports nutrition books or even contact a sports nutritionist to help you.

Finally if you repeatedly suffer from minor colds and ailments, make sure you take enough time off to rid yourself of the problem in the first place, otherwise you run the risk of regularly underperforming and risk picking up something more serious.