I'm on the path to becoming a particle system expert, and right now I'm doing a silly logo particle system. It basically forms my username out of particles, but I want the particles to stay in place until a desired time. E.g., I want to add a keyframe to switch the particle physics from no physics to newtonian (or other) physics.
You can animate the "Gravity" and "All" effectors under Field Weights of the particle system. Just keep them at 0 until you need them to affect your particles, then set them to the desired values to get the party going.
If your particles have an initial velocity you need to set keyframes for that as well.