I've had my 400 for two years with only regular oil changes. I've replaced hub fluid due to a ripped CV boot last spring. Does anyone have a laundry list of things to replace/adjust/check? I guess this is timely with spring around the corner. 62 in Appleton, WI today!

 

My manual's outside, and this is not by any means a comprehensive list (or even accurate, I am not sure how often you are supposed to do these), but if you really want to do a real tune up, you should include:
new plug
change gas filter
change rear gearcase fluid
change front gearcase fluid
hub fluid (if you haven't done both sides)
clean/replace air filter

Like I said, that is not a complete list, and some of those may not apply to your 03, but that should give you an idea of what to snoop around the forums for information on.