I generally use Path hold when climbing out to waypoints, Alt/Heading hold when I'm at the desired height and just flying about and Alt hold when orbiting an area looking for targets or waiting for JTAC to respond near the IP - something like that. Just pan down to the swich (or have it mapped to your stick), select the correct option and when your speed is above 200 hit the A key. If your speed isn't fast enough, the AP will disconnect as there is not enough airflow to keep the jet balanced in the correct configuration. The AP won't fight to keep you in the correct configuration if your speed is low when you engage it, but if your speed drops when it is already engaged, it will keep you at the right configuration.
To turn off AP, you can either manouever until it disengages, or just press the A key again. I use both methods (not at the same time), depending on what's going on!