1. Make sure you understand how to solve the 8 bags of gold problem. In working on this problem, try breaking the problem into a couple of cases,then working on each case. Keep careful notes of your work on each case to make sure that you've covered all the possibilities.
2. What's off the scale can be as important as what's on the scale.
3.Try to find a strategy that gives you the best first weighing, then think about the stategy for the second weighing (this is the hard part!)
Watch Terry Gaffney's 39 card trick for useful patterns.