I used to have little printed out cards I had made myself that explained Aspergers and how it affected Rachael. Then I could just hand it to parents when she was going to a party for the first time or meeting new people. Then if they wanted to ask any questions they could do so. Most people now know about it, so I don't use them anymore and when I have to explain to anyone now I am more confident in doing so. I am surprised by the amount of people who now know about the condition when I mention it.
The cards just simply explained that what the condition was, that it affected her social skills, communication skills and flexibility - Rachael is always very compliant in social situations unless she is reallyfamiliar - but it explained that she can be aloof or that she would have difficulty with understanding instructions etc. By doing this you aren't suddenly remembering that you have forgotten something and hoping that it is going to be alright until you come back!