Some of the local hotels have discount coupons. If you have AAA or a Military ID, you can show your card for a 10% discount on regular 1-day tickets. There are also 2 day tickets that are priced at a discount, or if you are visiting SeaWorld also, we have a 3 in 1 ticket that is good for unlimited visits for 7 days (including 1st day of use). If you are tying your trip in with a trip to
Disneyland, you may want to look at the Citypass. You can get one that includes the Zoo, SeaWorld, 1 day ticket to Universal Studios and a 3 day Park hopper to Disneyland - keep in mind that you have to purchase this ticket through Citypass's website, as none of the parks sell the one that includes the Zoo. Make sure when you receive it that it has a sticker on it that says "San Diego Zoo" to make sure you have no problems.
You might also check with your HR at work. Some are involved with a Company called Tickets @ Work, that sells discounted tickets.
Last but not least, you can do a google search for San Diego Zoo discount coupons - Make sure that you get the San Diego Zoo - not the San Diego Zoo's Safari Park, especially if it is a Living Social Deal - they are for the Safari Park only.