Man, maybe I've got some underlying issues, but if you're buddies are pulling it up, I'd say go pretty large. I've got a 40lb river anchor (Basically a bullet with 4 large flat blades that stick out from the bottom) for our 17' Wooldridge sled. I would say it's inadequate for anchoring in some holes on the sky, skagit, snohomish, cowlitz etc. I know letting out more line helps, but you just can't do that in a lot of places. Especially when you've got 4-5 people in there and they are all moving around like idiots.
The next anchor I get will most likely be the 60lb version. Just remind your buddies its a good workout.
It's also how you drop it down. I usually try to drop it down and then back down slowly till it starts to grab, also give it a quick jerk in the beginning to get it laying right. Not sure if thats that best way, but it's the best way i've found.