Too embarrassed to say how you feel? Write it instead.
Rather than saying goodnight, hand your date a letter. Tell her to wait to open it until you've gone. With this approach, you'll seem both romantic and mysterious. (Make sure to do the hand-off in a positive way, though, or she's liable to think you're breaking up with her via letter.)
When writing the letter, really think it through. Don't just sloppily jot down a few thoughts on a piece of paper torn from your notebook. Use the good stationery -- the stuff in your parents' desk drawer. Spend some time crafting your note on scrap paper until you've captured exactly what you want to say. Then use a good pen and careful penmanship so she doesn't think you "really bike" her.
Consider the tone of your note. Do you want it to sound upbeat, cool, or romantic? If you go too far over the top with the romance, you might scare her off and never make it to the next date. You want the note to be positive, but not gushing. It should be honest, but not fawning.
A letter might seem old-fashioned if you've grown up on electronic communication, but believe us -- it's far more romantic to read a handwritten expression of interest than yet another "U R cute" text message.