Hmmm. It's hard but there are a few tips I would share.

- Try and get on destination time as soon as you board, but don't stop yourself from sleeping if you really need it.
- Don't drink alcohol before/during/after a flight.
- Drink as much water as you can.
- Wash your hands before boarding and after deplaning.
- Take disinfectant wipes during the flight and keep your hands clean.

Over time I seem to have built up an immunity to aircraft-borne bugs but sometimes you just cant avoid it.