I agree with "walk confidently" but I find making eye contact, smiling, nodding, or even saying "hello" while you pass does wonders to subdue people's suspicions.
It's more the reason why you avoid eye contact. Looking nervous while shifting eyes to avoid eye contact looks suspicious. Avoiding contact because you are otherwise distracted thumbing through forms in a clipboard makes it look like you have a purpose for being there.
I agree, but it requires quickly reading the situation. Sometimes it can be as effective as not making eye contact.