I use mydatamanger and it is awsome. it does 90 day terms, forcasts usage and adjusts your daily limit based on data used /available.
I use this as well on android. I agree this is a great, free app for keeping track of data use. I find it fairly accurate as well comparing it to the public mobile data use (website) .
Another vote for My Data Manager, you can go into "My Alarms" and set custom data thresholds so you get alerted when you reach them. I have mine set at 1 and 2GB at the beginning of the 12GB period, ie; I know I shouldn't be hitting 2GB before 1/2 the month is over.
On iOS, if you also go into Settings > Cellular > Use Cellular Data For, you can see which apps are using data and how much. Also reset it at the beginning of the data period. Common culprits include anything that play video: YouTube, Facebook, Safari, Flipboard etc. You can either modify your habits or also temporarily turn them off so they're not using data, just wifi.