Because I run a university club, I have done this quite a lot. At the beginning of the year, you always get huge numbers of people arriving and it can be very daunting the first time.
I always do the following -
1. if you don't have a good uke it can be very difficult, so I try to get one from my own club to come along. Also if I can get other experienced aikidoka to circulate and teach during practise that is a great help (plus they are invaluable in getting people stood in the right place at the right time)
2. I try and make the classes a bit shorter so people don't feel they have to give up an entire evening until they are ready. Plus beginners have a shorter attention span usually.
3. I always start with a demonstration of a complete kata so the beginners see more aikido that they would just practising.
4. I concentrate on one or two base practises at first and work on techniques from grips -it is easier to get the correct ma-ai from a grip. I always show several applications of the same technique so I don't give the impression that everything is from grips.
Out of 60 students only 1/4 stay until the end of their course - you learn not to take it personally after a while.
Because a lot of people go to all the martial arts to find out which is best it makes for some interesting conversation!