Short and sweet is the way to go -- about an hour, scheduled when your baby is likely to be in a good mood, such as post-nap.
Invite only close family and maybe one or two very familiar baby friends.
Scale back expectations -- it's hard to know how your baby will handle the day. She may decide to go on a crying jag or skip her nap; she could sleep right through the event, too.
Pull out her favorite toys and blow a few bubbles -- that's all the entertainment you'll need.
Prepare your baby for the "Happy Birthday" song (nothing like a shrieking tot to dampen the moment) by singing it to her a few times beforehand.
Cut a tiny slice of cake, as it'll be a new taste to her, or serve her a cupcake, an ideal baby-size treat.
Consider her energy level before ripping into gifts. Your baby won't notice (or mind) if you open them later, and it's less battery- and assembly-induced stress for you.