I'm no stranger to anonymous sexual encounters; I actually prefer them most of the time.
Here are some pointers that should help you:
1) Qualify, qualify, qualify: Be totally frank about your preferred role(s), limits, expectations of duration, attitude regarding safer sex practices, etc. You do not want any surprises cropping up for which you hadn't prepared.
2) Get the person's phone number: Speak on the phone at least briefly before s/he arrives. If the voice sounds in any way fishy, cancel the rendez-vous.
3) Make the "date" when you have nothing else planned immediately after: Do not schedule a lunchtime "quicky" for the first time. Schedule several hours for the encounter.
4) Meet the person outside: If you have any sense of hesitation, you can always opt out.
5) Take your time: When the person comes in with you, spend as much time as you'd like discussing whatever you like. But don't feel rushed.
6) Do not attempt to broach any limits you have: Stick to your principles, especially as regards safer sex. Presume whoever it is is HIV+.
7) Maintain as much of a feeling of fun and play as you can: Lighten up! Have fun!
8) Don't be in a rush to clean and/or leave and/or kick him/her out when you're finished. Enjoy the afterglow and the conversation that might follow. | 
