From an interviewer's perspective:

-Dress one step above the job you want.
-Don't dis your old boss or old job; they'll assume you'll disrespect them, too. Find a simple phrase like "I feel the need to move on to better opportunities" and stick with it. Don't let them drag you into discussions about what's wring with your job.

From a practical perspective:
-Don't ever job hunt at work. Don't show up in interview clothes when you've asked for a long lunch for a "doctor's appointment." Beg or borrow a friend's fax machine to handle the resumes.
-No matter how tired you are, job hunt every free day. Pound the pavement. Talk to everyone you know (that doesn't know your boss or coworkers) and have a clean resume copy in your car at ALL times. Hit the online sites in the evenings after work and respond right away.
-Get a hair cut, permed, colored or whatever makes you feel great about it and buy a new interview outfit that you really, really like and feel smashing in.

Given your work history, you are most likely to find a better vet assistant position or in a entry-level career path like sales where they want eager and energetic and will train you. Not everyone is suited to those jobs though, and beware of "manager trainee" ads. Do vet offices have office managers -- it might be a step up for you.

If you have a chance, I'd get your teaching credentials anyway. If you do lose your job, teachers are almost always in short supply and even some substitute days would be better than no income.

BTW -- whereabouts? If you can type halfway decent, we may have a entry level office position opening up in So Cal.