Service Tax is to be paid on accrual or receipt basis whichever is earlier.
If any advance is received before providing any taxable service, then it taxable on receipt of advance.
Point of taxation rules says Invoice need to be issued within 14 days of providing taxable service, then taxable point will de the date mentioned in the invoice.
If invoice is not issued within 14 days then point of taxation will be the date when the taxable service is provided.
As rightly pointed out by "Ashish" there is an option to follow any of the two methods upto 30.06.2011.
For Professional people point of taxation rule does not apply ie they can continue paying service tax on receipt basis for the entire year.