If it's a 1 time import, the Import/Export utility in SSMS will probably work the easiest and fastest. SSIS also seems to work better for importing large data sets than a straight INSERT.
BULK INSERT or BCP can also be used to import large record sets.
Another option would be to temporarily remove all indexes and constraints on the table you're importing into and add them back once the import process completes. A straight INSERT that previously failed might work in those cases.
If you're dealing with timeouts or locking/blocking issues when going directly from one database to another, you might consider going from one db into TEMPDB and then going from TEMPDB into the other database as it minimizes the effects of locking and blocking processes on either side. TempDB won't block or lock the source and it won't hold up the destination.
Those are a few options to try.
-Eric Isaacs