Stacking one letter block on top of another gives you a points bonus, but the trade-off is that the two blocks take longer to get to their destination. If the block features a letter you know you need to complete the puzzle, make sure they get to the book as quickly as possible.
Although points are removed for moving a block forward manually, no such penalty applies when you push a block backwards. Exploit this to change the order of the blocks and make sure the really important ones are at the front of the pack.
Blocks occasionally become fatigued, but a quick tap will wake them up and send them on their way once more.