First, apply Bojote's tweaks:
http://www.venetubo.com/fsx.html
They really work GREAT and will do wonders for most of you. The only thing I'd rather not do is to use an external limiter. I prefer the internal FSX's limiter to 30 fps.
Then I always apply this nVidia Inspector for my nVidia graphics card:
If you don't mind to get you hands "dirty", then the AVSIM Software and Hardware Guide for FSX is a MUST
Appart from that, I always TURN OFF land and sea traffic, which hits very had on FPS. Also, I have to lower autogen to normal or lower in complex sceneries in order to avoid OOM