If reasonable, get a babysittter and book a hotel near the convention. These aren't necessary but will help you focus on the task at hand. I know moms who go as a group and leave the dads home with the kids and I have friends who use this as a couples getaway. Our kids are old enough that I want their input so we bring them and all of us stay near the convention.
At the minimum, make a list of the subjects you'd like to cover and your budget. If you have some specific thoughts on providers, jot them down too. Be warned that you may change your mind, but have a plan. As soon as you can get a list of vendors and speakers, mark the ones you want to visit and keep that with your list.
If you have some time, visit websites and stores (we love Mardel) to check out curriculum. This will help you get to know your options and the prices. Homeschooling friends are also great resources and will often let you look through their materials. Cathy Duffy offers helpful reviews on most of the top resources.
On the day of the convention, wear good shoes and be prepared to take notes for later. I know many people who browse on day one, review the materials that evening and buy on day two. Even if you have only one day at your event, I tell people to pack a lunch so that you can take a few minutes to leave the building, sit outside in the sunshine and regain your thoughts after a morning of information overload.
Lastly, know that if you leave without buying everything you needed, that's ok. Unless I really know that it's something I will use and/or the discount at the convention is significant, I'm ok with paying a little more later. I'd rather be certain that have unused curriculum at the end of the year because of an impulse buy.