First, restore a recent backup of the original_db as new_db

Then:

use new_db;
EXEC sp_MSforeachtable "delete from '?'"
(This basically just deletes all the data from the restored copy)
You probably have to run this multiple times for it to complete successfully, due to foreign key constraints, if you have any... so you can safely ignore errors like "can't delete due to FK_xxx"

I would probably also run the following on the database before you start putting more data in it.
EXEC sp_MSforeachtable 'UPDATE STATISTICS ? WITH FULLSCAN'
go
EXEC sp_MSforeachtable "dbcc dbreindex('?')"

After you're done, you probably also want to compact your database:
use master;
dump tran new_db with no_log;
use new_db;
dbcc shrinkdatabase (new_db, 4, notruncate )