Well, my guidelines are that the party area needs to be set up and clearly designated as such. All personal items and really fragile items should be put away. I like to make sure that everyone invited has someplace to sit and that there is a place to store coats, handbags, etc.
A bathroom for guests' use needs to be clean and ready with sufficient toilet paper, clean hand towels, soap, and all personal items put away.
I also like to have the kitchen sufficiently cleaned up-no dirty dishes from the previous meal, food and drink items that will not be served put away, etc.
There need to be enough drinks and refreshments for everyone (I also tend to have some extra).
All the setup needs to be ready by 15 to 20 minutes before the start time of the party, just in case someone gets there early. (If they're earlier, I will either send them away or ask them to help with the setup.)
At the party, I introduce guests who don't know each other, try to fill awkward silences by starting a conversation, and try to make guests comfortable in whatever way is possible within reason. If I see that a situation is getting tense, I do what I can to defuse it.
When it's time for guests to leave, I'll escort them to the door to say good night. If anyone needs a ride home, I'll arrange for it (including designated drivers when necessary).