Fixation of cells for long time storage - (Jan/24/2008 )

I am working on mouse splenocites and I’d need a method to fix and keep the samples collected to further analyses surface and intracellular antigens by flow cytometry. Does anyone know the best method for that, and for how long can I keep the samples before labelling? Which is the best method for permeabilising these cells?
I really appreciate any commentary or advice.

Thank you

we fix cells and have them in PBS (with little azide) and store in 4C. The samples are very well preserved and one can take these after a month and stain them as usual.