If you're referring to apps that counts your data for you out of your monthly limit, there are apps like Data Usage and Data Usage Monitor in the AppStore. Simply set your monthly limit, the date of the counter reset (a.k.a. billing cycle ends) and it'll gauge your data usage as you go along. As for tips on managing data, there aren't any apps for that. Just do a bit of research to find the estimate of how much data certain functions use such as retrieving emails, watching a Youtube clip, listening to music, etc. One word of advice, if say your data cap for the month is 2GB, 2GB is equivalent to 2048 MB, I would set the cap to 2000 or 1950 or something of the sorts so you have a grace margin so you won't get charge for another whole GB for using a MB more than your capacity.