I try to keep colours together as much as possible, and pay attention to both where you're placing the block + where other parts of it could potentially slide down a slope - try to let both influence your descision. I go for easy ish clears fairly often but generally try to focus keeping colours together and stuff. Use your peripheral vision, and because of the preview you don't actually need to take your eyes off the stack of blocks unless you want to look at the next one, which I'm not able to incorporate into my strategy yet anyway. As I play more I start to see more good and bad patterns of blocks emerge and more combinations that can lead to score-able shapes.