+1 on using a dedicated noise reduction program.
I use Neat Image it does the job on film scans very well.
I'd also caution against trying to eliminate it completely in any one area or things start looking a bit weird.
Hi Paul, yes point well taken. I forgot to mention in my reply that when dealing with skies, it is often best to put the sky on a separate layer and treat the noise on that layer alone; however in doing so care is sometimes needed not to let the noise reduction spill over as blurry stuff into the non-treated areas adjacent to the sky.