Second (not in importance just in the order I thought of it): Come prepared.
Theme and color are the most important ideas you need in your head. Bring swatches of bridesmaid dress fabric, ribbons from the bouquet and ideas about what flowers you will use - bring your laptop.
I guarantee you, if you google "pink cake" in google images, you will immediately have more ideas at your fingertips than you can count. There's some amazing talent out there. Find it.
Print pictures, pin it on Pinterest (WHAT? You're not on Pinterest??? Go...get an account and then come back in a few hours after you have become the newest addict...also, follow my boards, cuz I'm awesome and I pin gorgeous cake.)
Come to your tasting with all your ideas and your baker/decorator will help you sort through and find the things that will make the perfect cake.
If you came to me and said, "I want a steam punk cake." I would have no idea what you mean. But, if you came to me with a color scheme in mind an a picture like, say, this amazing one:
...well, I'd be right on track with you. And as a side note, I would weep for joy if someone ordered a cake this awesome. It would make me quiver with delight in anticipation of the project.
Also, be open to new ideas. Maybe this baker has a talent for sugar flowers *ahem* or is really good with some intricate piping you hadn't thought of before. Use their talents and ideas too.
Talking design while you're sampling flavors is a huge times saver. Why have two meetings when you could have one? And if you're wowed by the tasting, you can start working on the final design and sign a contract to reserve your date right then and there. Yes, that means crossing off something on your to-do list. Even better!
Cake Tasting Element #2: Bring your ideas and be open to new ones!