Planning a birthday party seating chart
A birthday is the one event where not everybody is sitting down at once, and your layout should say so. Plan the buffet queue and the standing room first, then put the tables around them.
Mix the table shapes
A room full of identical rounds reads as a wedding. A mix of shapes reads as a party, and it gives people somewhere to move to. Seat the guests who will settle in for the evening at the rounds, and leave the long table for the ones who will drift.
Put the buffet where the queue can form
Put the buffet against the wall furthest from the entrance and the queue has somewhere to go that isn't straight through your seating. Keep 8 ft clear in front of it. If you can serve from both sides, do: a two-sided buffet moves people twice as fast and it's the easiest fix there's for a jammed room.
Leave somewhere to stand
Budget about a third of your floor for standing and moving about. It's the space people actually use at a party, and it's always the space that ends up with one more table on it. If there's dancing later, that area doubles as the floor once the food is cleared.
Seat the tables, not the chairs
For an informal party, putting people at a table rather than in a specific chair is usually enough. It solves the hard part, which is who sits with whom, without pinning anybody down. And if a guest turns up with somebody you were not expecting, it doesn't break your whole chart.