It can be rewarding to take the occasional risk. For example, choosing not to take a partially-filled truck that is only attractive for yourself and revealing another tile instead in the hopes that another useful tile will come of it.
Taking animals that you don't have room for in your enclosures at the moment isn't the end of the world - especially if you still have the option of expanding your zoo. However, you should try to have as few animal types as possible in your barn.
The money actions offer additional possibilities:
You can sometimes choose to take an 'undesirable' tile of an animal type you don't want when you know that another player wants it. If that player buys it from your barn, not only will you have fewer minus points, you'll also have made a coin in the process.
Above all, don't underestimate the advantages of 'remodeling'. You can, for example, use timely exchanges to make money multiple times.
Example: Bert filled his 4-space enclosure with flamingo tiles on a previous turn and received a bonus for it. He has 3 chimpanzees in his 5-space enclosure.
On Bert's next turn, he decides to carry out a money action. He pays 1 coin to the bank and performs an exchange: he switches the 4 flamingo tiles in the 4-space enclosure with the 3 chimpanzee tiles in the 5-space enclosure.
Since there is now an empty space in the 4-space enclosure, Bert can use a chimpanzee tile on a subsequent turn to fill the enclosure a second time and receive another bonus. Bert is now also only one tile short of receiving a bonus for his 5-space enclosure.
Since both enclosures now have only one empty space left, they will both score points at the end of the game.