Sheet out and really force your front foot into the straps when you feel a catapult is near. If your back foot is already in the straps it will generally save you as you can pull up on it to get lower and drive the board forward with the extra power.
Generally I get catapulted when I'm in the straps during a long lull and when I'm leaning too far forward to try to keep speed and end up getting catapulted by a gust due to my unstable position,
so keep scanning the water for incoming gusts. Although this requires constant mental effort (being focused 100% of the time), it will save you physical effort as every time a gust catches you off guard and you get catapulted, it's that much more effort to uphaul/waterstart again.
Finally, although you want to be in the harness as often as possible to increase your time on the water, being in the harness while not planning and not in the straps can easily lead to a catapult if you're not paying attention to the windward water for signs of gusts. So if the conditions are really gusty try to get the board planning and stable and then get into the harness.
Hope this helps.
JP Super-X 106l, North F8 Ram: 5.8m,