concurrent updates problem

I have on a web application
update threads set views = views + 1 where forum_id = 1 and thread_id =
1
just to record view for that page - stupid I know but people want it.
Problem comes when I use apache's ab to test it.

It just doesnt produce the correct result with 10 or so concurrent
connections. The views ends up being completely out.(no failed queries)
I read up on transaction Isolations , using select for update etc
but can't get it to work correctly - I thought serilizable might do the
trick but only few queries were rolled back -
Can anyone point me in the right direction to solve this ?