My deck is about 15 years old. I believe the previous owner stained it only once, when it was built. I've pressure washed it, and now am getting it ready for re-staining. However, a lot of planks have cracked surface, pictured below. What should/can I do to prepare areas like this for re-staining?
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!