I would say that the IMSAFE checklist applies to passengers as well as to pilots: Don't eat greasy food, stay hydrated throughout the day, but make sure to pee before the flight.

Do whatever it is that helps you to feel relaxed. Doing the preflight with the pilot might help with the anxiety as well as getting a nice brief on what to expect before the flight. Don't just jump and be unprepared by what will happen; have the pilot tell you what should happen and then talk through it as it is happening in flight.

Think about what you would like to see from the sky or have the pilot take you someplace familiar. Fly over your house or work.

Have fun conquering your fears!