Mind, in terms of leaving wood unpainted, what may work just fine with lumber that was cut in the early 1900s may NOT work well with typical lumber from the early 2000s. A lot of the very durable unpainted old barn siding is from old- or at least natural-growth wood, rather than plantation pine that has been pushed (genetically and culturally) to maximum growth rate. Wood that grew slowly also generally *weathers* slowly, because the growth lines are mashed close together so there are not large swathes of the weaker more porous summer wood exposed.
I think my main point here is that people reading this thread (not the o.p, to whom this doesn't apply) should realize that even though yer grandfather and great-grandfather could put up wood siding and leave it stark starin' naked to the weather and have it last 100 years, that does NOT imply that YOU can do the same. Unless you have an unusual source of wood.
Although FOR SURE the construction features that darkmatter cites will make a big difference in the longevity of any wooden structure.
Just a thought,
Pat