http://headache-after-workout.blogspot.com/ -- How to prevent an exertion headache after workout :

1. BE Hydrated: Drink enough water that you're piss is white the day before your lift and the day of, and drink water during your workout.

2. NEVER Hold Your Breath: Most good lifters pride themselves on having good form. When I'm working with a client, in addition to proper mechanics, I include proper breathing as a requirement for a rep to count. "You're holding your breath, that rep doesn't count."

3. Maintain a NEUTRAL SPINE: Unless you're performing a neck exercise, there is no need for your neck to be bent. Let it be comfortable and neutral and focus on the target muscle.

4. Allow for RECOVERY/ do CARDIO: First of all, do some cardio to condition your heart and lungs to be able to work hard. This will prevent the body from being too shocked on rep 9 and 10 on those Squats! Second of all, give yourself enough time between sets and exercises for your heart rate to come down.