Display top N records with a group

Here's a pared down simplified example of what I'm trying to do.

I want to run a mySQL query from PHP to give me a list of the top salesman
for each fieldoffice.

I have a table called SALES with the following fields:
FIELDOFFICE
SALESMAN
MONTH
SALESAMOUNT

The following query will give me results for all combinations, but I only
want the TOP salesman for each office, not all of them.

SELECT *, SUM(SALESAMOUNT) AS totalsales FROM SALES
GROUP BY FIELDOFFICE, SALESMAN
ORDER BY FIELDOFFICE desc

I know I could process the entire recordset and just output the first
record in each break level, but I'd like to do it with one simple query.

The LIMIT parm won't do what I want... I looked at DISTICT and couldn't get
that to work with the sum ...

Thanks for any help you can provide.

T