For me, there's two components - getting a mob's attention, and getting them to keep that attention.
I'm currently using a Bio/ tank for herding; her AOE taunt aura picks up mobs nicely. Just run up to/through groups of mobs on the upper hull, and they'll start attacking and following you. On my Rad/ I have Incarnate powers, so I usually start my gathering-up with an Ion Judgement lightning-bolt, which can hit up to 40 targets.
So now you have guys following you... the problem I used to have was that they'd "fall off" of me, so I started doing corner-pulling. Corner-pulling is where you get their attention, as above, then use geometry to break line of sight; don't get too far away, or they might just return to their spawn positions. This is good for two reasons: one, if you're out of line of sight, they're not shooting the crap out of you from halfway across the hull. Second, they gather into a nice knot right where you are, which lets you reapply taunt easily, or slap a debuff on them, or whatever. All over the hull, along the sides, are places you can do this (the little radiator fins that line the walls the bomb-grates are set into work fine).
Once I get them close to the edge, I hop down onto the ramp. The bowl ramp has a "lip" on the ramp where you can break LOS again, and the Rikti will knot up again if you pause right under that lip. Then I hop down into the bowl; right around the floor of the bowl is another lip you can break LOS with, and they'll follow you down and your blastery friends can nuke the heck out of them.
This is all anecdotal, mind you. I don't know the inner mechanics of how and why they break off versus stay on you when pulling; I've seen some guys pull a whole wing of the hull in one shot, and they all stick to him just fine (HI, REACTORN!).