Seaside is easily my favorite Dominion expansion, my wife and I got Prosperity from "Santa" and it's already her favorite. We always "draft" our ten cards and she dislikes ending up with more than one or two duration cards in the set of ten, since they really do change the game quite a bit. Intrigue is on its way, and I'm sure we'll nab up the next expansion when it comes out, too, whatever that might be, so she's relieved to see their tendency to pop up on the table thinned out a bit.
Initially we just had Dominion and Seaside; I would deal out about half the deck of potential action cards to each (this is what we use the blue-backed cards for), set aside a few randomly chosen, face down, and randomly choose two more, face up. Then we'd go back and forth until we had ten on the table. Furthermore, each player chose a card to "ban" one active card after each game - those two cards cannot re-enter the draft pool for three games.
We actually do the same thing when playing with friends, since we both have practically every card memorized, and those who play it are barely familiar with it. ;p This allows us to get the game completely set up before anyone arrives.
Because Prosperity introduces new victory/treasure card elements, though, I'm changing up the system a bit. We'll still do the draft, and each person will always have a pool of 20-25 cards to choose from, but we rotate what sets are available in each round, as follows (skipping Intrigue games, of course, till it shows up any day now):
Classic
Classic + Intrigue
Seaside
Seaside + Prosperity
Classic + Intrigue + Prosperity
Intrigue
Intrigue + Seaside
Classic + Seaside + Prosperity
Prosperity
Prosperity + Intrigue
Classic + Intrigue + Seaside
Classic + Seaside
Intrigue + Seaside + Prosperity
Intrigue + Prosperity
Classic + Prosperity
Classic + Intrigue + Seaside + Prosperity
The new victory card and gold card only appear in games using Prosperity in the draft, and they are removed if less than two Prosperity cards were selected. We've done away with the three round ban, too, since it's so much harder for a particular card to pop up several games in a row. I definitely recommend it for couples, though, if you have one or two sets instead of three or four. Great way to reduce the "omg I can't believe you picked Embargo AGAIN" grumbling...
Insanely obsessive? Wellllll yeah. But Dominion is all we play right now; this helps keep it fun, competitive, and fresh every game (we NEVER see the same 10-card layout anymore, each play environment is unique - this is what keeps us coming back to it night after night). I'm not terribly embarrassed by it.
edit: I completely forgot I have an actual
question: Do any of you ever play with the Ambassador card? Is it just defense for a curse-heavy game, or something? I have never seen it played and a cursory examination makes it seem pretty lame. I've been trying to come up with some sort of decent tactical combo for this guy for months.