I put up 28' trusses on 12' walls a few weeks ago. Takes a few guys to get the gable up, after that it can be done alone, though its slow. Nail cleats along the wall tops to serve as stops and give you something to nail the trusses to, then you drag the truss in upside down, carry one tail up a ladder and set it on the wall, then carry the other tail up and set it on the other wall, the second tail takes some real grunt! Even them up between walls. Now take a 12' pole or 2x4, wedge it under the peak and push the truss into a horizontal position and stick the end of the pole on the ground, it'll stay. take a 16' 2x4 with a short fork nailed to the end, stick it in the peak webbing and push up and over vertical til the truss is leaning on the previous truss. Note that the first pole will fall as soon as the weight is off of it, so avoid it. You must brace each truss as you go. I used those pre made galvanized spacers to space the top chords. Like I said, its slow, and only feasible with 2x4 trusses. But its no more dangerous than other methods, and free.