Here's a useful tip for any of you who use China Mobile. If you want to change your monthly data allowance, send an sms to 10086. In the message, just type the numbers 661. You will get a return message in Chinese detailing current rates:
1. 5元/月 30MB
2. 10元/月 70MB
3. 20元/月 150MB
4. 40元/月 700MB
5. 50元/月 1GB
6. 70元/月 2GB
7. 100元/月 3GB
8. 130元/月 4GB
9. 180元/月 6GB
10. 280元/月 11GB
Then just send a message again containing the number of the plan you want, i.e., 5. Then you will get a return message containing the details of your current allowance and asking you to confirm the change to the new allowance. Then, just send a message with the character 是.
That's it. Your data limit will come into effect from the next month, and you can change again any time you want. To check how much data you've used in the month, send an SMS to 10086 containing the numbers 66.