If you get desperate, and I've been there, you can issue a CHECKPOINT command, backup the log, issue another CHECKPOINT, and backup the log again. That frequently clears the tail end of the log quicker. But it doesn't always work. Sometimes patience is the only solution.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning