First, center your runs around your players' characters and their story; whether the 'story of the day' happens to be an evolution of a PC-Contact interaction, that's up to you, but quite often it can be, particularly if your players are of the sort to have background between the character and the contact. Take, for example, a current character at my table; his mentor (and talismonger contact) not-entirely-recently lost a point of magic due to prior run events. In the not-so-distant future, I could have him call up the PC and ask him to go to, say, Tir Tairngire to get some paranormal plant or critter part to use in a ritual to gain that point of magic back.
As Spooky says, though, as often as works for your table/players. Remember that plots don't happen solely within the bounds of one (or several) missions; they expand outwards, taking place during (supposed) off-time, during another run, or whatever. Eventually, you can get to the point where you can have a contact call up in the middle of a mission and it quickly becomes a question of which to do ...