If you absolutely have no choice -?????
then, make yourself as unappealing and unapproachable as possible
Wear horrible clothes.
Contort your face slightly
Paint a front tooth black
Wear BenGay
Eyebrow Pencil in a bit of a mustache
Blow your nose a lot.
Speak in a slow, distracted, whiny voice
Ask them to repeat everything they say a few times
Just say uh,uh,uh, like your in middle of davening and point to a prepared shopping list.
If your picking up the family pizza store order, call it in, and have a piece of paper with your family name on it, and point to it.
Otherwise any of the recommended methods from all the posters here will totally work – if you have your ipod, phone, tehillim, Binah magazine handy.