How do I figure out the lowest IFR cruising altitude for my flight?

It is the altitude at or above the MEA which is consistent with the direction of flight. For magnetic headings between 0° and 179°, you must cruise at odd thousands (5000ft, 7000ft, 9000ft, etc). For magnetic headings between 180° and 359°, you must cruise at even thousands (4000ft, 6000ft, 8000ft, etc).