Best index structure for table that holds a large amount of log data

Folks
I have a ASP.NET solution that needs to log every time a method is invoked.(that part I have done)
I am logging this activity into a table
I had constructed a table that has a identity filed that is also  the Clustered Primary Key

When we do load testing and the number of clients is greater than 20 I get errors (which I also log to another table).
The error is "The underlying provider failed on Commit"
Inner Exception is: "Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding"

I also SQL profile these tests

Under 10 users inserts are under 1500 msecs
Over 20 users inserts are about  6000-9000 msecs
At 50 users inserts are over 30000 msecs

This table is never read by the application and it has only one index the clustered index.

So how can Index this table to reduce the insert time and avoid this exception?
Or should the table have no index?

regards

GregJF