Register early. As in, check the ride web site on January 1 to either register or learn the earliest date you can register. This ride always sells out, but lately as consensus has built that it's the best organized multi-day ride in the state, it's started to sell out even earlier. Select the bus/bike return ticket option, unless you want to stay additional days in Vancouver BC and return via Amtrak. Make your first day Bellingham Days Inn reservation at the same time, if you don't want to go the inexpensive route and stay in the WWU dorms (the latter will eventually sell out but you don't have to make your reservations right away). Make your second day reservations for either the Buchan Hotel, or (if cost is no object) the Coast Plaza Hotel. In the past the latter has sold out pretty quickly.
Mount two water bottles on your bike frame, rather than just one: there aren't as many water stops as there are on the Seattle to Portland ride, and it's easy to get a bit dehydrated. In my experience there hasn't been a need to keep a bike lock on the bike, since unless you fall really far behind there will generally be other riders at any place you'd like to stop.
Don't forget your passport, or your bus/bike return tickets.
If you're a slower rider (as I am), make sure you start on the first day when the start line opens, and bring your headlight (you can put the headlight in your luggage on your second day to save weight).
If you're a slower rider, then before going to bed the first night pick up some food for the next morning. Plan on having breakfast and still leaving the earliest possible time you can drop off your luggage. The idea is that it feels really good to be able to pick up your luggage, take a shower and change clothes, and still make it to the party before things start to wind down.
The morning after the ride, there's really no point to being on anything other than the last passenger bus (or even waiting in the line when you can instead have a seat in view of the line): I don't think the bus with the bikes has ever arrived before the last passenger bus, at least any of the times I've ridden RSVP.
For the trip back, make sure you have your passport with you rather than in your luggage or on your bike.