Seed them with the selective antibiotics you used previously

I supplement the culture media (which already contains pen/strep) with gentamicin, during the first/second weeks after sorting

Depending on the type of cell sorter and the conditions of use and maintenance you may be abl to preserve sterility. In any case, it is a good idea to add gentamicin as Diana suggested, during a few days (3-4 media hanges or 2 passages). Most cells tolerate well gentamicin.

Another suggestion is to have more serum in the medium where the sorted cells fall and avoid centrifugation for 15-30 min after sorting. 

Good luck

With Selection marker Such as G418 for Neomycin Etc.