I barfed in my full face in a race at whistler once, and have done it on many training rides. Everytime it was due to the following recipe:
Step 1. Eat a hugh meal whilst on the vino
Step 2. Drink booze for further 8 hours or more (the longer the better)
Step 3. Sleep for 3 hours in a comma
Step 4. Eat left over curry, kebab or choclate and drink litres of water just before riding/racing
Step 5. Push ones self to the your physical limit (hit your max heart rate and keep it there within minutes of finsihing the drink) with no warm up
Step 6. Barf your guts up
Step 7. Feel rubbish for the rest of your ride
Step 8. Eat a hugh meal
Step 9. Get back on the booze to celerbrate
It has a 70% hit rating on the barf. You can increase the rate by playing with food and alcohol combinations. My buddy has a 100% hit rate these days, it took him years to perfect. It also helps if its really hot.