When dealing with deleting data from tables which have foreign key relationships - which is basically the case with any properly designed database - we can disable all the constraints, delete all the data and then re-enable constraints.
-- disable all constraints
EXEC sp_msforeachtable "ALTER TABLE ? NOCHECK CONSTRAINT all"
-- delete data in all tables
EXEC sp_MSForEachTable "DELETE FROM ?"
-- enable all constraints
exec sp_msforeachtable "ALTER TABLE ? WITH CHECK CHECK CONSTRAINT all"
More on disabling constraints and triggers here.
if some of the tables have identity columns we may want to reseed them.
EXEC sp_MSforeachtable "DBCC CHECKIDENT ( '?', RESEED, 0)"
Note that the behaviour of RESEED differs between brand new table, and one which had some data inserted previously from BOL:
DBCC CHECKIDENT ('table_name', RESEED, newReseedValue)
The current identity value is set to the newReseedValue. If no rows have been inserted to the table since it was created, the first row inserted after executing DBCC CHECKIDENT will use newReseedValue as the identity. Otherwise, the next row inserted will use newReseedValue + 1. If the value of newReseedValue is less than the maximum value in the identity column, error message 2627 will be generated on subsequent references to the table.