Good advice above & not much left to say.

But I'm a Mudcatter so I can't hold myself back.

I agree with leeneia on ideas for changing, and agree that those sorts of changes should be frequent.

Then along comes Tom and brings up tune sets. I've deliberately followed "Waitin' for a Train" with "The Dying Hobo," BECAUSE OF the similar situation, but I do it because hopefully you can wring all kinds of pathos out of the first one and then break it up with the comic relief of the other one--if anybody's wondering what's hilarious about a dying hobo, this is the one that ends,

His voice grew weak, his head fell back, he'd sung his last refrain;
And his partner stole his coat and shoes and jumped a westbound train.

I've also followed "Wabash Cannonball" with "City of New Orleans" for the opposite contrast--railroads up; railroads down. I need a key change in there, but that's a simple G to C (in my case) resolution.

I think what I'm saying is that you can add "theme" to leeneia's list of categories (time, M/m, speed, vocal) and get another dimension to play with to produce "variety."

Good thread. :)

CC