1. Make sure your stored procedure is not recompiling every time it is run. (RECOMPILE = ON).

2. Check the index fragmentation using sys.dm_db_index_physical_stats.

3. Check out this post on that wait type http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=790196&SiteID=1.

4. I assume you know what the stored proc is doing, but just in case run a Profiler trace to get an idea of the processes being executed.