We'll get a certain number of tabs per day (I believe four or six, unless you won the additional trading tabs via RSP). Each tab is good for one visit to the board. During each visit you are given one minute (60 seconds) and you can exchange any two of your pins for any two pins on the board. There is usually a mix of LE, LR, old event pins, HMs (new and older), and OE pins. Many pins on the boards will be AP/PP versions - at least half of everything I pulled during Love Is An Adventure was AP/PP. There are several lines with several boards, and each line faces the back of the board, so you can't see what's on it until you're standing in front of it during your minute.
There will also be a partners board for Best in Show people with pins from DSF and some international parks (no WDI though, boo).
At the DSF events, I always use the strategy of pointing to a pin I see that I'm interested in as I scan the rest of the board, since they are very large and busy boards and it can be easy to lose track of where pins are on it in the anxiety of the moment. I also like to start looking from the bottom up, since I feel that most people start at the top so it gets more picked over.
As far as what to pull from the board, I always try to take things that fit into my own collections first, unless I see something that is clearly going to be an exceptional trader (like marquees, higher end PTDs, etc.). That way I'll be happy with it no matter what. If I don't see anything I collect, I'll go for a popular character (Ariel, Belle, Rapunzel, etc.) that will possibly be a good trader. If I still don't see two pins that fit those, I'll just go for something that looks cute or cool or neat. You aren't obligated to trade two, but I figure if I've waited in the line for that long I may as well, and I can always use what I pull to trade back to the board on my next visit if I'm really unhappy with it.
And ultimately, unless I pull a scrapper that another trader has exchanged in, whatever I walk away with will probably be an upgrade over what I trade to the board (hidden Mickeys, booster packs, etc.). So I don't really stress too much over it in the long run. I only stress when I see more than my allowed trades and then I have to choose between them.
Another tip - bring extra Mickey backers with you. Often you'll trade a pin with one post and pull a pin with two or more, and the cast member doesn't always have spares handy.
