IMO...
#1: Deal with it in the realities of NOT being Planescape. Don't write it in cant, don't parade planar arrogance, don't assume that DM's won't still want to use their own homebrew alongside Sigil. Write it from a Material-Plane-Centric stance. Figure out what it adds to that (fer instance, an easy base for planar adventures).
#2: Don't counter-act the PS stuff. Sigil still has factions, still has the NPC's that it has, still is made up of the wards, etc. You don't need to explore these in a whole lot of detail, but what detail you have shouldn't invalidate what came before.
#3: Sigil should be like Saltmarsh or Union: a detailed city filled to the brim with plot ideas and motivations. Sigil's plots and motivations exist in large part already. Translating them to 3e and advancing the time-line on these would be the biggest key. Motivate the PC's to explore the planes and consider the philosophical factions via Sigil.
#4: Take into account the Faction War. Let us know what's been happening since then. In other Faction news, tell us about what they do on the Material Plane, how they interact with the standard D&D Greyhawk set up. If I add the Athar to my campaign, am I going to get St. Cuthbert's Inquisition?
#5: Use Sigil's place as a possible haven for low-level characters to your advantage. Either Sigil itself, or as a launching point into the planes.
That's a short preliminary list.