The first one is a minor one that is easy to figure out (but might not necessarily be worth it):
When playing the "back side" of each area, you will have to solve a riddle to pass. If you talk to the skull think enough, you will be offered a hint for 1,000. Take the hint, then reload, and you can get the hint and save your money.

The second one is less obvious, but is a much bigger money save. When you purchase the Maid Outfit for 99,999, don't save the game, or at least not over your main file. When you reload, you will have the Maid Outfit (since it carries over to other save files), but still have the money. (Of course, you do eventually run out of things to spend money on, so this trick isn't quite as useful as it sounds, especially if you plan on junk farming.)