Keep an eye on this forum -
http://www.gamefaqs.com/boards/gentopic.php?board=933195
Especially Wed Thurs and Fri. People will post threads with a subject that says something like "Turnips selling for 550. 99k entry fee". If you have turnips to sell, keep the page up and keep refreshing so you can spot any new threads that appear and get in early. Sometimes long lines can form quickly. This is easier if you have a laptop and can keep it next to you while you play.
In the first post they will give their friend code and any rules, like no running, stay on the paths, etc. Then you reply to the thread with your friend code, and maybe the number of trips you need to make (you can carry up to 1500 turnips at a time in your pocket).
The person who posted the thread will add friend codes in the order they were posted, sometimes only 4 or 5 at a time to avoid a traffic jam. Sometimes they will post to let you know you were added, sometimes you just need to keep watching for their gate to show up.
When you go over, head straight for Nooks, don't run, stay on paths if they have them. If you sell a full pocket of turnips, Nook might give you as much as 800,000 depending on the price. Your wallet goes up to 99,000, and the rest goes into bags of 99,000 bells each. If they are charging a 99k entry fee, just drop one of those bags at the entrance before you go out.
Most people prefer to keep WiiSpeak off when doing business. Basic etiquette is to ask before buying anything, picking anything up, taking fruit, etc. People who open their towns to strangers take some risk because there are people who will steal hybrids, cut down trees, or misbehave in other ways. They want to be able to keep an eye on everyone, so don't wander around on your way to Nooks and back.
When have made your last trip and are all finished, let the town owner know so they can add the next person if they are only doing a few at a time.
This is a good way to make a lot of bells, but it takes a little work to stay on top things and get into a turnip thread before the poster closes up shop. We haven't bought turnips in the last couple weeks because 10 million bells is enough right now. However when we do have turnips we manage to sell for 500+ every week no matter what our own Nooks is offering.
Ian