How do I prepare this cracked deck wood surface for staining? (Picture)
Ouch, that looks like a splinter waiting to happen to someone barefooted. You won't like the answer: sand, sand, then sand some more. A belt sander with coarse paper would take it down to smooth pretty quick, then you could switch to a vibrating sander for your medium and fine grit sanding steps.

You might consider renting a commercial type drum or vibrating sander normally used when refinishing hardwood floors, depending on the size of your deck. I would definitely consider it to save my back!