Cassiel - Taking the plunge is a big thing - Congratulations and welcome. Are you asking for practical suggestions or a more spiritual aspect? For practical, a couple of thoughts. Depending on how your priest does the baptism (in our church we use a holy horse trough), you should pay attention to the outfit you get baptized in. If it's a cotton t-shirt and white pants or a white cotton chemise, make sure that when it gets wet, it's not going to be see through. You might do a test run, and if it's too thin, wear another t-shirt and slip underneath or your going to run the risk of looking like a spring break wet t-shirt contest. I've seen some rather embarrasing, though unintended baptismal garment choices.
If you paint your toes, make sure you touch up the pedicure before the big day. This seems petty, but if you're standing in church barefoot with groddy, rough, calloused feet with spotty nail polish, you're not going to be thinking about spiritual stuff.
Sponsors/godparents should remember to get a gift for the newly illumined. An icon, a nice Orthodox book, or purchase the baptismal cross. A gift isn't a requirement, but it's a nice thing to do for your spiritual child. A sponsor should be an important part of a catechumen's life and this should continue after the baptism. Like remembering their name day, the anniversary of their baptism, prayers for each other and spiritual advice and instruction. My god mother was the first person I ever met in an Orthodox Church, and helped me to learn the services and the small-t traditions that I was unsure of. (Anyone out there, please remember in your prayers my godmother, the departed servant Helen. A truly Godly and wonderful woman.)
Just a few things I can think of now.