When deciding where to place your habitats, try placing very popular animals (Orcas, for example) Near the back of your zoo. If you do that, and then place attractions (Like photo booths) food stands, gift shops, or anyplace where a guest can spend money on the path that leads to your Orcas, then on the way to the Orcas the guests may spend money on these places. Try it out with Orcas, Lions, or Tyrannosaurus Rexes.
You need to save money or you will lose the game because all the animals will die if they are not well fed. It has happened to me before I used all my money on the animals and they didn't have any food or water.