You can get a pretty professional looking appearance for a relatively low cost by designing a snazzy banner in Illustrator (Photoshop will have gigantic files) and having Kinkos output it for you. An 8x4 full color banner will cost between $200-300. Hang it in the back of your booth and you'll draw a lot more attentiion than using the white-with-black-ugly-block-letters sign that's provided for free.

Also, check in advance what, if anything, is provided with your booth. $75 to rent a crappy $10 plastic chair, or $150 for a crappy wooden banquet table with some stapled-on plastic skirting for 3 days is not uncommon from the bandits that provide booth furnishings; we usually go to Walmart and buy some chairs and just leave them at the end of the show, it's much cheaper.