Like Neo says, the bigger the rocks the bigger the gold with exceptions of course. Look for clay layers that are acting as a false bedrock and have bedrock concentrations of gold on them. Also, sometimes its best to just take everything you think you know about where gold is gonna be and forget it and just sample pan anyway. I have been surprised on more then one occasion in finding gold in places it shouldn't be. For example, everyone and their brother will tell you to look for hardpack and I usually say that too. But I have found some crazy gold in some loose fluffy light sands that you would not think gold would normally be in. And I mean crazy gold too, 40+ pieces to the pan which is great for the east coast where I am at.
Also pay attention to the gangue you are finding along with the gold. Both iron oxides and mineralized quartz are well known for running with gold, but often times gold will be found along side other deposits such as lead(galena), copper and tin (cassiterite).
And do your research on local geology and try to find mineralized fault lines, and drainage basins for such. Look for contact zones along the intersecting bodies of two different country rocks (For example slate and limestone).
That's all I can think of for now, hopefully that helps.