You should put your degrees and awards on the wall behind you (although it looks like you have a nice window behind you, so maybe the wall to your left). Putting your credentials where your visitors can easily see them conveys confidence and power. It subconsciously conveys that you are in charge in your own office; here I am, and here are all my credentials and experience (literally) backing me up. Put your personal photos on the wall behind your visitors - they are for you to enjoy. Non-personal art or photos (like a landscape or something) should go where you both can see, and will put your visitors at ease.
The placement of your clock is also important and will vary depending on your position. The ability to see the time without being obvious about it (ie turning your head to see the clock) is a power thing. If you want to give your visitors that power, then put the clock behind you. If you want it, put it behind them. If its mutual (like if you're a psychologist or in HR, for example) then use two clocks so you both can easily see the time.
Put a mirror behind your door or in another inconspicuous location so you can check your hair and make up before you have a visitor or a meeting. Try to avoid too many overly decorated spaces, because that can make you appear frivolous. For example, bookcases should have mostly books, not pictures or knick-knacks. In fact, knick- knacks should be avoided unless they are relevant to your job. Like a tchotchke with your company's logo or something. If you are in HR or a counseling-type position, put a box of tissues close to where visitors will sit and maybe put a bookcase next to their seating area. When they need a moment to gather their thoughts or compose themselves, they can distract themselves by looking over at your book titles. Important files should be close to you, so you don't have to get up and walk across your office to get information. Don't forget to make sure visitors can't see your computer screen.
I know some of this sounds silly, but I had a co-worker who was an expert in conveying messages with office space. I totally agree that your office should be something that makes you happy to walk into. You spend a lot of time there.
To the extent that it matters, I'm an attorney, but I almost never meet clients in my office. Most of my clients are larger entities who I travel to meet with, or I just work with via phone/email. So I'm less concerned about communicating power/time/etc., and more with being comfortable for the 8 million hours of my life that I'm there. The attorney offices in the firm are generally more personality-filled than might be average. I still try to keep pictures to landscapes, but it's not a really stiff place.