For quick opening, el wristo flicko does the tricko. For reliable opening, Senor Thumb y Senora Circle are the shizzle.
I believe the tips that follow are a mishmash of stuff from a Marc MacYoung book on knives and a Peyton Quinn book on self-defence with maybe some Loren Christensen thrown in:
Practice using your thumb and opening like normal. Use a mirror and or recording device (or friend who enjoys brutal honesty whom you won't attack) to search and destroy any unnecessary movement you make when drawing and opening your folding pocketknife. For reliability and practicality, you're better off removing the self-made obstacles that slow you down instead of hurtling faster over those obstacles. Taking a further point from MacYoung's book, Effective Offense, your subconscious mind is more likely to accept a technique that's fast because it's stripped down than a technique that's fast because of complex methods. Just something to keep in mind when dealing with high-stress situations.