One thing I do when closing for the season, " I do remove all equipment and store in garage except for heater" I cap or slightly plug the outlet or inlet on heater with a rag to create stronger suction and use a fairly large shop vac and suck remaining water out of the tubes. They say you could as extra precaution add antifreeze to the lines but there really isn't anyway to keep it from running right back out unless you cap them off which I would not do. I also use a custom made cover for winter for my particular heater which you may or may not need, My house heat pump does not get covered but then it has to run year round. As stated on another reply is to keep vents clean , probably the most important maintenance step.
