My method is to get a long piece of 1/4" allthread, a couple big washers, a couple small washers and some nuts, and a large piece of scrape steel (a little larger than the back of your receiver).
Drill a hole in the steel. run the allthread thru it and put a nut on the end.
Put this against the back of the receiver with the allthread up thru your front trunion.
Slide you barrel assy down over the allthread and line it up with the trunion.
Put a couple washers over the end and then a nut.
Turn the nut slowly with a wrench and it will put the barrel assy right into the trunion.
I do all of this with the receiver in a vise, and i've allready dressed the barrel end and the trunion.
With this method, you can go so slow that when you are close to lining up the cross pin, you can't miss.
I've done this with my first 3 builds with no problems at all
Here are the pics of those builds:
http://www.ar15.com/forums/topic.html?b=4&f=51&t=80325