some good tips on this thread, I like the Irwin quickgrip clamps idea, may have to try that, but they do sell bead clamps for that purpose as well (but the quickgrips are a lot cheaper!).
my contribution: HF sells tire irons pretty inexpensively, I have 3, <$10 ea on sale. very useful on stubborn tires, and they are flexible enough they won't snap on you. resist the urge to use large screwdrivers, pry bars, or crowbars...
the machines can either be electric motor driven or air motor driven. they will all use air for the clamps and inflation, but if you get one that is exclusively air driven, it will use A LOT of air for the motor (think D/A sander or sandblasting). you need a compressor that can keep up or you lose a lot of power trying to spin a tough/stuck tire, so keep that in mind.
lube is your friend! there are lots of homemade recipes for making lube, but I just use a can of spray lube from walmart...easy and doesn't freeze.
motorcycle tires can be done, I did my Harley, but had to take off the sprocket, however, they make adapters that fit on the rim clamps that raise the wheel up to clear the sprocket. name brand are expensive, ebay has no-name ones much cheaper, and a guy on here a while back posted a set he welded up himself...
buy valve stems and other consumables/bead sealer, etc. in bulk. you may think you only need the 2 pack from walmart, but the minute people find out you have a machine, you'll be mounting everyones trailer tires, snow tires, etc. and doing someone a favor than having the valvestem snap off or leak without a new one to put one is no fun...
I also keep a grinder with wire wheel at the machine for buffing out the rusty/corroded rims, and a few hand wire brushes as well as a good rubber mallet.
get a good valve stem installer, core puller, etc. the cheap ones aren't worth the hassle...and good ones aren't that expensive. and put a high-flow inflator end on it if it doesn't have one. helps greatly inflating to seat the bead. the little round ones that clip on you see all the time don't flow enough...Milton makes a good one.
same with tire plugs. buy a good kit with the good 2 pc. aluminum plug installers. why? because when people know you can do tires, they'll bring their grandmas old tire over with a nail in it at 9am on a Sunday morning before church and expect you to fix it since there are no more gas stations that are open and repair tires anymore...