Data Enabler Widget (
link) to switch data on/off when you want to.
That is what I use. My wife used 5mb of data the first day at work (first time leaving the home wifi with her phone for extended period of time.) I put the link above on both of our phones. She hasn't used a single bit of data since and can turn it on if she needs to use data for something.
I just leave my data on because I'm on wifi at work as well. I only used 10mb the first month most from leaving work's wifi area while updating something. You do have to watch though even being on wifi because I was downloading a 25mb app and the wifi signal must have been weak and it switched from wifi to data.
Also, make sure other things in the settings are set correctly if you don't want to be using a lot of data. You can set so programs only sync on wifi or update on wifi. I estimate my data using less than 2mb/month if I don't specifically use it because I have the network locating turned on for my weather widget to give me current location weather.
In case you don't know, as long as you keep it active each month with the minimum data package, the unused data (and texts) roll over. I initially put the $25 500mb data on my phone and just renewed for the 2nd month with the $5 10mb data. I expect this to last me at least 6 months, probably more 8-10 months, before I put an additional 500mb on.
Using 100mb per month and adding 500mb on every 5 months, would cost around $120 for the year. That's around the entire cost of a contracted smartphone for a single month, LOL. I LOVE prepaid!

Especially since I don't expect to come anywhere close to using 100mb per month because I don't live my life with my face constantly behind a cell phone like the majority of society seems to.