Couldn't agree more with the others who say that most au pairs would rather be in a smaller room with a nice, kind family, than in a bigger, more souped up place with nasty employers.
Our au pair was in a fairly small room with a single bed, but had her own en suite bathroom.
We gave her a netbook and she used this way, way more than the TV. She skyped her parents every evening for free via it and used it for all manner of communications. Definitely an idea if your budget allows it.
The only other essential tip is just to be nice. This person is looking after your most important 'possessions' in the world - why on earth wouldn't you want to ensure that that person is happy, secure, content and seriously inclined to be nothing but nice to your children?
I can't imagine the mindset behind the sorts of people who like to wield power over their au pairs and exploit them - it just doesn't make any sense to me.
I made it clear to our au pair that her main job was to look after our (initially just) DS - if there was any spare time above and beyond this when he was sleeping and she was on duty, sure, do some housework, but that was not what she was employed for.
Random gifts, just because - even just a chocolate bar with the shopping. Doesn't have to be extravagant. Remembering, noting and marking significant dates in her calendar, giving her unexpected time off, and basically just appreciating her and thanking her and noticing when she does stuff.
This is all the important stuff. Other than that, draw up a contract beforehand and outline the sorts of things she would be responsible for and be sure she is OK and clear about them all and on board. After a week have a chat and compare notes - are you happy and is she happy? Anything that could be improved? Agree to get any issues out in the open and sorted quickly so that they don't build up and become worse. Chat with her, talk with her, get to know her and make sure she's OK.
A mutually beneficial relationship with someone you live and work with is an utterly fabulous thing. And just remember that au pair mean 'on par' - she is not a servant or in some way beneath the rest of the family - it works best if she is 'on par' and part of the family.
Good luck!