You are provided only a limited number of hints in the game, and after that you'll need to either earn more by completing stages or with purchases. To save yourself that trouble, make sure to use hints only when you really need them. Most early stages can be completed in a few tries, without hints, and that allows you to save them for later when they're most needed. A lot of times, you can also figure out how to clear a stage just by looking at the resources you are provided and figuring out how they might work together, so cheats are often unnecessary until the stages near the end of a given pack.
Secret Levels
Throughout the game, you can access four secret levels in each level pack by performing one of the following actions.
Find every medal in the associated level pack.
Find the missing shamrocks.
Purchase the secret levels in a given pack for $0.99.