I read a number of threads on the subject on ReefCentral. DSBs are typically designed to meet the goal somebody has. For example, a goal might be worm production; this goal would suggest a different type of DSB than if the goal were nitrate removal. The summary of them all (assuming the goal is to decrease nitrates):
-Install it downstream from the skimmer so that it is as close to 100% clean, detritus-free water as you can get
-Have strong, non-turbulent flow. So, laminar, straight-line flow that keeps detritus from settling into the sand bed.
-The deeper the better, so a 5-gallon bucket is great.
-Keep it in relative darkness
-Use sugar-fine oolitic sand (aragonite). Ron Shimek (mud-guru) recommends 0.125mm average particle size. This is hard to find but I imagine it's out there from an online vendor somewhere.
Hope this helps.