1 - Never buy power or range boosters, always take the interest.
2 - Never upgrade a tower - $ for $, you can never buy as much power upgrading as you can by buying multiple towers.
3 - I've never purchased a green or purple tower.
4 - The biggie - You need to use the fewest amount of towers possible in each level. The more money you have in the bank when you send in the next wave, the higher your score.