Here are a few things you should keep in mind before you start planning. Do something that will grow with your child. Matching that very cute and cuddly crib bedding you chose is great, but will it last? If it's your first child then doing a baby nursery can work out great if you plan to move each child to a different room, keeping the nursery for each susequent baby. Be careful with personalized items being too babyish. Ask yourself, "will this still work when the baby is three? Five? Seven?" Unless of course, you have the time, desire and money to redo a room every few years!
Think about adding family heirlooms or things that have sentimental value. I've seen some absolutely gorgeous rooms come together based on one little piece or trinket that was passed down from someone in the family. Imagine what you could do with an old baseball glove that belonged to your grandfather or a mirror that was once in your great-aunt's home? This teddy bear room came together with the help of teddy bears collected by mom and heirloom furniture passed down from Grandma!
Also try and think about the function of the room. Storage is crucial for most people as well as a place for reading or playing. Think not only about the theme, but how the room will be used.
Lastly, your imagination is your strongest tool! Look around you and draw inspiration from things around you. You never know where a great idea might come from!