Learn the Land:
The first, most general, thing you can do is to get to know your local landscape. Use the maps on this site to find where the local streams and drainages are and where the roads you drive the most intersect these potentially dangerous areas. It is especially good to know of hazardous bridge areas and low-lying roads near creeks.
The single most important thing you can do for your safety is to be mentally prepared for a rapid onset flood event. Make no mistake about it, when a large rainfall hits Boulder, it can turn this area into a "war zone" and you won't know how bad it's going to get once it starts. There are always complicating factors like poor visibility and/or the darkness of night. Keep in mind that every rescue effort draws precious resources from a very limited supply. This means that if you make a poor decision that results in a minor rescue, you could be drawing resources away from those in greater, perhaps even life-threatening, need.
Hurry Up and Wait!
Sometimes the best thing you can do is respond rapidly to keep yourself from immediate danger, then switch gears and "shelter in place" - waiting out the few hours it takes for the situation to stabilize. Resist the normal urge to rush home immediately. Chances are, you won't really know how bad things are until its too late. Conversely, many flash floods only present grave dangers for a period of hours before subsiding.
Use your cell phone to locate family and insure that everyone is safe where they are. But don't linger to avoid overloading the network needed for recue and relief personnel during a large scale event.