The Rebel Wing men for the trench run, what did they really expect to accomplish flying in that formation in the trench? Wouldn't it have been much more effective to fly above the trench giving the leader some space ahead to protect him, like what Han does in the end?
This actually has a partial in-film and EU explanation...
The Death Star's designed mostly to defend against large-scale assault, and EU writers ran with the concept that their surface guns are much more effective against fleets of capital ships over starfighters, and that the higher above the surface, the more likely of hitting and creating a net of laser fire to do damage to Rebel fighters comes into play then.
This is sort of confirmed with how the Rebel Fighters are evading the surface guns then and then fighters are ordered out. This pits everyone into a bit of a fair fight...
EU's sorta hinted at other fighters then running interference at a higher altitude... This draws some cannon fire and Imp fighters up to meet them, giving the attack run pilots a little more of a fighting chance... You could come in straight at the exhaust port, but would it be the most heavily defended portion of the trench ultimately? So coming from that higher altitude, you could possibly get tore to pieces...
So how does Han do it? I've always then assumed Han kind of "slips in unnoticed" somewhat. Everyone's tied up with fighters above, fighters nearer the surface, and the gunners have all these blips and nobody notices the Falcon... Hell, it may even not show up on sensors making it all the more stealthy or something.
Han buzzes in, clear's Luke by throwing his own interference into the pursuers, and peels off immediately (again lending some credibility to the concept that from his angle he's a sitting duck... otherwise why not stay and make sure Luke's ok?), Luke does his thing, they head off and the end...
One of the things about the "higher" rebel ships has hinted at early model A-Wings being at the battle too, and ultimately doing some of that higher buzzing around taking out Imperial fighters and drawing some of them away from the core group.
Of course a lot of this helps if you buy into the notion that there's more fighters at Yavin than the battle in the film ultimately implies. A fan edit of ANH really added a lot of ships, and again some EU have added some more ships. The film's hangar and all the scrambling pilots certainly implies a lot more were there who never make a peep (another couple squads with their own task?) anyway.