Please take this as possibly being helpful rather than trying to make you feel worse.
UCLA medical school did a study.
They focused on two groups of people. One group said, "If there is a bug to catch, I'll catch it." The other group said, "I'm too busy to get sick, I rarely catch anything."
They split the groups and put half of each in one of two rooms. They told both groups they were in a room full of cold bugs and they wanted to see what they would catch. One room had germs and the other was germ free.
After spending a day in the rooms. The group that said, "I always catch whatever is there to catch." showed cold symptoms regardless of which room they were in. The other group didn't come down with symptoms regardless of which room they were in.
The reason I point this out is that you have already said, "If I fly more than 3 hours, I catch a bad cold." I'm not sure any technique will work more for you than changing your attitude to, "I'm on vacation and I'm not getting sick, period."
There was a recent study that expensive placebos showed better results than inexpensive generic drugs. The point was that your expectations really can become reality, if you think the more expensive drug will work better, it will (even if it's a placebo).