turn off cell data whenever connected to wifi, or if the cell signal is so weak its useless (~2/5 bars or less, 3G or 1x).
Monitor your carrier signal strengths around your residence and neighborhood . Areas of weak coverage, I turn off my cell data. This also saves battery power. My city has really weak cell coverage, locals keep voting down cell tower construction measures. opensignal.com has some pretty accurate coverage maps for the places I frequent.
The drawback however at least on Verizon, is I won't get voice mail messages. But thats a very minor negative for me, and its almost a moot point anyways as weak as the cell signals are. I am lucky to get 4-5mbps down... I can forget about uploading. Not sure about your carrier, but Verizon uses cell data allocation from my plan to ping towers and merge in. The spottier the coverage, the more data it uses. Its not much... but it is data that I pay for.
There's a setting where you can force apps to update only over wifi.
log into your carrier and monitor your data usage closely. Verizon has a really good data usage widget, like a pie graph. So at a glance I can see what my device data usage is.