SWOP has some excellent guidelines for how to be an ally with working girls – SWOP is Sex Workers Outreach Project, a group formed to advocate to end violence against sex workers. The Australian chapter describes their mission is described as:

SWOP promotes the health, safety and wellbeing of sex industry workers in a way which enables and affirms their occupational and human rights and is the leading agency in NSW for HIV prevention amongst sex industry workers.

There are a number of SWOP chapters world-wide, so for useful resources including information on sexually transmitted diseases and personal safety, make sure you checkout their sites.