I have a 150mb plan with Verizon, and have no trouble avoiding overages. I have two weapons in this task:
the app Quick system Tasks provides an easy way to toggle all radios. This makes turning on and off data, wifi, gps very easy. I have QST on my quicklaunch, and i can access it with only one swipe.
The other (for verizon, anyway) is a bookmarked site: Verizon MW3.0. When connected over 3g this will give you an update of your data usage. All carriers should have some equivalent of this.
Personally, i would prefer to not have any data plan, and only use wifi. I want a smartphone, but i don't care about always staying connected.
addiarmadar is right though, if you aren't going to, or don't want to, be diligent about your data use you should pony up the extra $15 a month and get unlimited data.