Your biggest factor as far as earning massive amounts of points in Diamond Dash is how many of the diamond bombs you earn and use. Every time you clear off a combination of diamonds you fill a bar at the top of the screen, and you drop the bar down every time you click diamonds that can't be cleared. Fill it to he top and you will earn a free diamond bomb which clears two entire lines off the board and earns you almost 15k points. The more of these you earn per match, and use, the easier it is to rack up massive points.