6. Use your hints sparingly, and only when you need them

You are given a certain amount of hints when you start the game. If you use them all too quickly, you'll have to buy more if you need them. I've found if you use them sparingly, you rarely ever need to buy them since you're given a few here and there throughout the game. I've only used them on the more challenging levels in each level pack, particularly the last few levels. Even then, I always try a level several times before even considering using a hint on it.