This one of the most talked about trick on the internet. All you have to do is to restrict the network to WCDMA mode.
I recharged with 98 and hoped to get 3g speed but i did not get the speed. But soon i realiized that my balance was below 50.......as soon as i recharged my account, i got 3g speed!
So whenever doing this, make sure you have-
1. above 50.00 balance
2. 3g availbale in your area
3. network restricted to UMTS/WCDMA/3g mode