Here's my take on statements:
*In a nutshell, describe what you're doing and how, with a hint of why. Too much explaining of your motives runs the risk of steering the viewer away from exploring their own interpretations of your work.
*Use first person in most cases.
*Keep it accessible; not everyone digs artspeak. Share a draft with a non-art friend. Is it clear to them?
*Read it aloud to yourself. Are you out of breath at the end of some sentences? Long sentences often aren't as effective as short ones with strong action verbs.
*Don't overuse adverbs or semicolons. Avoid parentheses.
*Watch out for echoing words -- the ones that keep popping up repeatedly in a paragraph or even in a single sentence. And if you're using a really unique word, only use it once.
*Don't forget cadence. Be sensitive to the rhythms of your phrasing. Reading a statement should be like looking at good art. It should be a pleasure.
For bios, use third person and pull in things like education, where you live, gallery affiliation, awards, etc. Think of it as condensing your resume down to 100 or so words to provide a snapshot of who you are as an artist both in a tangible way with important milestones and a "this is what I'm all about" way.
What I found most helpful was browsing gallery websites that post statements of their artists. Find examples that seem to have your voice/tone and a structure you like for the intro/middle/conclusion. You'll see some lousy ones out there but keep looking. A great example can be the best teacher. Good luck with it! And if you want someone to read them over for you, I'll be happy to
