Alex Cornell maps some ideal seating arrangements for dinner guests. Some tips:
7 Person Rectangle: It’s very easy to get screwed in this scenario. While it may appear like you can sit anywhere except the ends, this is not so. You are at risk of sitting next to the lonely end-seat, which requires you to speak soley to that person for the duration of the meal.
2 Tables of Any Size: You’re fucked. Regardless of how you time your approach, you will inevitably choose too soon. Lament as the other table’s attendance crystallizes into what is clearly the superior group. Sometimes it’s best to visit the bathroom while seats are chosen, so any seating disasters are the result of chance, and not your own miscalculation.
A large version of his map here.