heres 2 ways.
1- you can right click on you iphone in itunes and hit transfer purchases. that should do it
and if it doesnt try # 2
2- Download all the apps through iTunes that you have on your phone,then sync. I know it acts like you have to buy them again but it wont charge you I promise
also be sure you have it check to have your apps synced or it will delete them off the phone.