The easy answer is don't anchor. But holding up on a spot takes more experience than anchoring. The best tip was the one for chain. Make sure you have an adequate length of chain. If the boat's in the mid-20' range and up, going to 3/8" chain isnt a bad idea. I'd definitely have 20' of chain on a 20' cc. If you dove and looked at your anchor, you'd like to have enough chain so that somewhere around half is always in contact with the bottom. Having run dive charters on a 26' boat, I had a 20lb anchor and 3/8" chain. I could pull up on a spot, lay out the anchor, tie it off and 90% of the time it didn't move. All while watching people try and anchor for 10 or 15 min. I don't remember hanging a wreck. Having said that about the amount of weight you now have out, an anchor ball(and not the smallest one you can get away with) is a must. Not only for the ease of pulling, but it also pulls it away from the wreck.