First off I am not advocating riding without a helmet.
That being said, current helmet designs do not offer much, if any protection in high speed crashes, and in some cases even low speed crashes may be more serious with a helmet on than without, due to rotational forces.
(i.e. like a whiplash as your helmet glances of a hard object)
Still I prefer to wear my helmet, it might not be the most becoming piece of attire but I still feel more secure with it on. (Not to the point that I would take extra risks thinking the helmet will protect me from my own stupidity, because I know it wouldn't)
In my opinion the two most important cycling saftey tips under any cycling condition are:
1. Keep a well maintained bike and don't ride it if it needs servicing.
2. Ride smart, be aware of your surroundings and be as prepeared as you can be for evasive manouvers.
After that proper protective clothing appropriate to the cycling environment.