Standard practice in the group I work with is to leave a small offering that we feel is appropriate: if we're not sure, our standards are copper pennies or a few strands of hair. Several of us keep small stones or biodegradable substances handy for offerings. (I keep undyed spun wool, when I remember.) Some people use tobacco, which has a long tradition in many Native American traditions (and which is therefore quite appropriate if you live in those areas) but finding unpolluted tobacco is not necessarily simple if you don't smoke.
Thanks, I never thought it would be something as simple as pennies and hair.