1. Start Early
You can't leave this job to the last minute, as much as you want to put it off. As soon as you have most of your RSVPs, or even before then, start mapping out your tables.
Be aware that things will change so you need to give yourself ample time to make adjustments. And also give yourself a few days in between each change to scream and throw things. Then go back to it.
Your suppliers - like your caterer and your stationer - will need to know final seating a good few weeks before the day.
2. Map it out
Go low tech for your first few goes - use pencil and paper, or sticky notes, before graduating to anything more permanent, and definitely don't go to print until you really have to.
3. To Group or not to Group
Lots of people will make sense to group together - uni friends, distant family, colleagues - but there will always be stragglers. Resist the urge to put all the odd people on one table unless you have to, instead try to fit a few people in with current groups that you think might get along. Your neighbour couple might have the perfect sense of humour for work friends, your cousin and his wife you've never met have a lot in common with some of your friends from school - you get the picture.
4. Modern Families
Whether it's divorced parents or people who have had a falling out - you have two options. One is to figure out a seating plan that means those people won't have each other in their line of sight during dinner and are as far away from each other as possible, OR tell everyone to suck it up for one day and play nice as a gift to you (they also still have to give you a gift though...).
Be amenable and considerate to an extent, but if you find yourself bending over backwards for a handful of particularly difficult people, lay down the law and issue an ultimatum - sit where we put you or eat outside...
If you are having kids at your dinner, depending on the ages you might decide to put them all together - give them their own kid's menu and some table games.
If they are at the table, remember to ask the parents if you need to get them a highchair from the caterer.
6. Elbow Room