It is very very useful trick for them who changes their google id from GOOGLE PLAY Again and again so if you want to switch your google account in GOOGLE PLAY and if you do not want to RESET PHONE SETTINGS then you can follow this tip to add or change google account from GOOGLE PLAY.
Instructions:(You must have a rooted Phone)
1)Download ES FILE EXPLORER or ROOT EXPLORER from Market
2)then go to INTERNALMEMORY/data/system/ and Delete Accounts.db file
3)Restart your phone and Done.
