It's All in the Flow - Using less furniture to keep your "open floor plan" open is a good start to having good flow. Make easy pathways in your space. Weird furniture placement that hinders how people move about the space is something designers notice right away.
It's All in the Size - Use of proper scale of furniture and accent pieces is important. Having too large of furniture for a room or too small of a light fixture in a room, can really throw off the balance. Sometimes correct scale of something will determine if the piece belongs in the space more than the style of the piece. Who cares if the bed is the perfect Victorian style for a Victorian room if you can't even walk around it comfortably because it's too big!
It's All in the Lighting - Poor lighting can make a room feel closed in and even creepy at times. Using the correct lighting in a room makes a big difference in how a designer might feel if they were in your space. I mean, poor lighting affects us, doesn't it?
it's All in the Mess (smell, bathroom)- Large amounts of clutter, bad odors or a dirty bathrooms are HUGE issues for designers in a space. We all can say we have visited somewhere that might have had all or one of these issues. Look at your space with objective eyes (and noses) to make sure you aren't falling into any of these terrible traps. You can have the cleanest house on the block, but if it smells bad, it will feel dirty anyway.
It's All in the Theme - Using coordinating colors and styles will keep your space from feeling off. Too many colors or styles can make a place feel messy or cluttered--even if it isn't.
It's All in YOUR Style - One of the last things designers pick up on is a lack of the owner's own, personal style. You can have a picture perfect room, but without some of your personal style it can come off as cold and sterile. Add your personality in a room to warm it (and potential buyers, guests and designers) up!