When considering your seating plan, take out your master list of guests and divide it into logical groups (your parents' friends, his parents' friends, friends from work, friends from high school, relatives, etc.). Guests will go so far as to rearrange place cards in order to be with "their group," so you might as well work with this tendency, rather than trying to fight it. The separate lists allow you to see roughly how many seats you're likely to need for each clique.
Wait until all of the RSVPs for a given group are back (including the people you have to phone to ask if they're coming before you start assigning tables to that group). If you start early, you'll end up reshuffling.
What do you do if a group contains 12 people who accept, and a table only holds 10 people? Split the group evenly in half, six people to a table, and put them at adjacent tables. Fill in the other four seats at each table with "unmatched" people: the couple from a former job who don't know anyone else, the cousin whose parents didn't come, etc. The partial group will get conversation started, and there will be enough "extras" that they're likely to feel comfortable joining in.
If you act on your seating plan while you still have a few RSVPs out (for instance, people who haven't returned your calls), start with eight seats to a table rather than ten. The extra two seats enable you to drop in compatible people who RSVP late. If you end up leaving those two seats empty, a table of eight is still full enough to spark conversation. You are, by the way, better off with two tables of eight than with one table of ten and one of six. A half- empty table is depressing to everyone seated at it.
When you encounter people who don't fit readily into a larger group, seat them with people who are likely to be compatible. People of the same age or in similar fields are a good bet. Single people who have come alone and who don't know anyone should be seated at tables with as many other single "dateless" people as possible. If this means your best friend from high school is seated at a "relatives" table with your three handsome, single cousins, that's fine. Arranging people so that they're likely to have a good time is far more important than maintaining strict protocol boundaries.
It's up to you whether you use one place card per person or per family (or couple). Either way, you run the risk of "losing" someone if you rearrange tables without checking your work carefully. With one card per family, it's easy to count the people you're moving as "one" rather than "two" or "three" or however many are represented by the card. With one card per person, it's easy to move Cousin Mark without remembering that his girlfriend Brandy Smith has to be moved with him.
When you write out place cards, think seriously about using first and last names rather than titles. There are two reasons do be a tiny bit less "formal" with the place cards. First, many married couples with the same last name don't really like to be addressed as "Mr. and Mrs. John Smith." Unless you're very confident about your knowledge of your friends' preferences, you're likely to irritate some. Second, at least some of your guests will be seated with strangers. It's very helpful for people making new friends to have a written name in front of them. It would be ideal if John and Sharon Smith's table mates remembered them as "John and Sharon Smith," not "John Smith and… what was his wife's name? Suzy?"
To direct guests to the right table, the simplest method is to put table numbers (or names) directly on the place cards, then set out the place cards on a table. An alternative, if you've used the one-card-per-person scheme, is to put all the cards for the couple/family in an envelope with their family name and the table number on it. Make sure the table numbers are prominently displayed on the tables. Do not expect 200 guests to find their tables solely by checking a schematic posted in the reception hall-you'll have an incredible traffic jam around the schematic. If you have place cards on the tables, you still need a little card for each couple/family telling them where to sit.
Put the place card table in a prominent spot near the entrance (or after a receiving line), but don't put it directly in an entrance where it can create a bottleneck. One slow guest can create quite a traffic jam. Given a choice between a square table and a long thin table, take the long thin table! A long table presents a greater area to the guests, allowing the savvy ones to slip in, grab their seating information, and slip away.
It's possible-and increasingly popular-to have favors that double as place cards. Some couples put the place cards in tiny picture frames. Think about personalizing favors with the guest’s name, just be sure to be obvious if you choose to do so, so that guests don’t sit down in the wrong seat not noticing the personalized favor in front of them.