Sigil, being an urban setting, is the perfect opportunity to make use of those social skills (and if you have the PH II affiliation rules).
I almost always have various portals detailed for an adventure before the PCs go looking for them. So, to give an example, lets say that the PCs need to get to the Dwarven Mountain on the Outlands.
I start with two or three portals that will get them there or into the surrounding areas (such as there is a surrounding territory on the Outlands). I do multiple portals because the social interaction is important in my games, and the PCs can fail or succeed based on their actions. Usually there will be a hard one, a non-difficult one, and an easy one.
I create a gather information table that details various things, including portal locations. When someone goes out and uses gather information, they can roll and get clues as to who knows where these portals are. So in the case of the Dwarven Mountain, they might end up being pointed to an old dwarven wizard, the bartender of a dwarven bar in the Lady's Ward, and an adventuring group who recently visited that area and has been telling everyone about it.
So then the PCs go off, use Diplomacy, Bluff, Intimidate, or whatever and interact with various people. The dwarves in the bar might challenge the PCs to a drinking contest, which would call for Con checks in my game. The dwarven wizard might want the PCs to deliver somethiing for him while they're there, and the adventurers might just want some money for the information. The PCs' actions will determine which comes about.
Then I'll note which group the PCs interacted with and try to use them again later. Maybe the adventuring group wants to go somewhere the PCs have been, and they come to the PCs later asking for portal information. The dwarven wizard, after they completed the mission for him, and then he'll ask them to do something more dangerous for him, or the dwarves at the bar will invite the PCs to go on a quest to take back their ancient dwarven mountain kingdom that a dragon has taken from them (oh wait, that's been done...).
That's how I work with my Sigilian urban campaign.