Unless you are the boss, people are checking for signs that you are doing things. Keep your desk messy. It looks like you are referencing notes. Reply back to easy e-mails quickly. Emails with the most people CCed on them get highest priority. After all, we're looking to cast a wide net of people seeing that we are "doing" "things." When you get up to walk around the office, keep a prop. A simple document in a folder works wonders. Make sure to walk hurriedly and reply to questions from stray co-workers with "Haha. Staying busy!" or "Never a dull moment. That's for sure." Keep it brief and vague. Call a customer support number and don't press any numbers (or choose the one that has the longest queue). Having a phone up to your ear keeps co-workers at bay. Have many programs open on your computer. Pretty much anything that makes you look like you are drowning in work.
Pro Tip: change your wallpaper or standby timer to an hour or greater. That way it never looks like you have been away from your desk for too long. Also, never ever talk to co-workers about what you are doing, not even your "friends." Word gets around, and you do not want Judy from accounts receivable tattling on you.