from what i've heard, being there is a way of guaranteeing you'll get in, whereas sitting at home on the internet is somewhat of a shot in the dark...
volunteer at the event and give yourself a reason to go see it in person before signing up the next morning.

Being there, if possible, would take the anxiety out of registering online. IMWI sold out same day last year, I think I heard 40 minutes after opening online.

Online registration is 10 a.m. local WI time the morning after the race on active.com. The sight gets hammered and it is sooo slow. I had 2 screens going and was still freaking out, it took 20+ nerve wracking minutes to process the information.

Are you feelin' lucky....

When you sign up the morning after the race - you will receive a voucher - you will then complete registration on-line two weeks later using the voucher number you received. You pay when you complete registration on-line not the day you get the voucher.
Pretty much whoever is in-line gets in - athlete, volunteer whatever.
Pretty much a finishers shirt, medal, bumper sticker is it. You pay extra for the pasta dinner. Once you arrive there for the actual race they walk you through the whole process from registering, to practice swims etc. Not much swag but a well run event. The day after the race they will have finisher bike shirts etc, so have some $ set aside for that if you want that or other finisher merchandise - they ran out last year very quickly. And more $ for pictures of course.

Yep. Registration will be on Monday morning. If I remember correctly, registration onsite started at 9:00 am, which is an hour before the online registration opened.

Also, I would scope out the hotels in the area while you are there and then make a hotel reservation immediately after you register. All of the close hotels fill very fast.

As for staying Monday, you might want to do that anyway, since there is a thank you meeting for all of the volunteers Monday morning.