I actually have a question similar to this on the board right now =). I have a way of doing it but was wondering if there is a better one. I can tell you what works when playing a Tank (which really can be a lot of different classes depending on build). Basically this:
1. Put some sort of protection on yourself (via using hero or whatever)
2. Flag H/H back (like really far back).
3. Aggro with bow.
4. Use AoE or something to get a hit on every mob, also, let them hit you a few times.
5. Remove flag for H/H.
Step 4 is key. Due to aggro rules the fact that you have damaged a mob and that they have damaged you, and that you're right next to them should help hold aggro a little on the tank. Incidentally, this same strat can be used with a hero tank ... it's just a little harder. Read this if you haven't:
http://wiki.guildwars.com/wiki/Aggro
Keep in mind that this works well when fighting a lowish or medium dps group of mobs. When fighting a group with high dps/elemental damage you'll want to send in two tanks at least and/or use more prot skills and/or spread out damage some.