You might want to put people next to people they know, or you might want to split them up a bit so they meet new people. But try to put each person next to at least one person they already know. People are more likely to get on well if they are similar ages or have similar interests. It might not be a good idea to put your ’alternative lifestyle’ friend with the piercings and tattoos next to your 80-year-old grandmother. A little common sense goes a long way.
Generally you should try to put families together and work colleagues together. But if you know people don't get on, try seating them separately. It is worth breaking with tradition to have a stress free event.
Avoid mixing age groups too much. Young children should be seated with their parents. Older children can be seated with their parents, or on a table together.
Try to create balanced tables, with even numbers of males and females. It is traditional to alternate male-female-male-female guests in some cultures. Some business dinners are seated male-male-female-female for variety.
If it is a group of people that know each other well you could try splitting up married couples for extra variety.
Try to avoid putting guests on the same table as ex-partners, unless you are sure this is OK. Remember that every room has 4 corners!
Resist the temptation to have a ‘leftovers’ table of all the people who didn’t fit on other tables. It is probably better to distribute such guests evenly.