To go north, a convenient place is Innherredsveien, around where it crosses Mellomveien. You can get there by crossing one of the bridges bridge east of the train station, Bakke Bru (bridge) or Nidelv Bru. Inherredsveien becomes the E6.
To go South, you can intercept the E6 at Heimdal, near the train station, by taking a train or city bus south.
Klet is maybe a better option. Right by the motorway, 5min waiting time in the winter. Bus 71 (not sure) runs couple of times a day from the center of Trondheim and by the stadium.
Alternatively, you can follow the signs out of the city for the north, walking through industrial areas and then follow the signs past a roundabout for the south towards Oslo. There's a nice lay-by. Ask to be dropped off at Bratsbergveg or anywhere along the E6 out of the city. Don't try to walk further along this route as you just get to a tunnel which continues for more than a mile..really dangerous to breathe in all those fumes. Eick! Jass waited here for five minutes in August 2010 (after walking all the way to the tunnel and cursing..walking back and losing over an hour..). Also, close to this spot is the 'largest autonomous syndacalist anarcho community in Europe'..self proclaimed, I suppose, but still quite large. You should be able to find a place to sleep if you follow the road winding up north past the coffee factory. You'll see the graffiti on the right hand side. They're incredibly unorganized and not so helpful, but someone should have some space, somewhere..