It is best to work with fresh tissue.

Process your tumors into single cells, dye with your antibodies and then you can preserve cells in paraformaldehyde for later analysis.

I have also heard you can sometime preserve first and the do the dye but never tried it myself.

I imagine you would be able to deep freeze samples in the right freezing solution (as you do with cell culture) but you would have to take in account the loss of cells and cell populations during thawing.