Points
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.
Stacking
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.
