Hitchhiking out of the city is pretty simple, there are park and ride services that connect the main city centre bus station with car parks by the entrance to the M11 motorway.
For heading north on the M11 (towards Peterborough, Leicester and The North), get the Madingly Road Park and Ride service all the way to the end. From here it is around 15 minutes walk to the entrace to the motorway north.
For heading south on the M11 (towards London and the M25 motorway, get the Trumpington park and ride service all the way to the end car park. From there it is about 5 minutes walk to a large layby where cars can stop (just before the roundabout) - I hugely recommend either having a sign saying London, or hitch on the southbound slip road, but it is quite a fast and dangerous place to hitch (nowhere for cars to stop).
User:Whisperingofthestars has hitched out of Cambridge many of times to London(latest March 2008). Trumpington has taken 10 minutes of waiting. Other than this, from the north of the city, it can take a little longer. If possible and hitching from above cambridge, try to get a ride to the south where you'll find more traffic going to London and beyond.