1) Open your broeswe and search web page or any things in Google then instead of closing your browser as usual, search number like 200000000in google and make a call to that number. You will get message like "you cannot call to this number." But, still try calling it again and again unless the GPRS is automatically stopped by operator. Now your call will be ended then gprs will be stopped. Now close your browser and check your balance. You will get balance same as before.
2)This trick may harm your mobile so it is preferd not to use. Still if you have no problem in your mobile evenwhen you take your batteryoff without switching the cell, you may still try this trick. Use applications like opera mini or mig33 or any other internet access application. Use it as much as you like. But make sure it is not disconnected. Later, when you want to close the application, dont close it with the menu option or red master button. Just take outyour battery from your mobile without ending gprs or application. Then keep your battery out for about half an hour. Then insert battery, open your cell and check balance. You will not be charged for your data usage. Thank you.