The Manhattan Association of Realtors put out a great list for maximizing a buyer's first impression when they are viewing your home. Simple, yet effective, these are my top three:
1. While the agent is opening the lockbox, the buyers have time to notice details around the doorway. Make sure this area is neat and tidy. Apply a fresh coat of paint to the front door and frame.
2. Your home should be welcoming. You want to buyers to picture themselves being able to live there instantly! Each room should look bright and clean. Consider painting darker rooms a lighter color.
3. Closets will be opened and that junk room will be seen. Think about how you can make your storage areas tidier. Maybe even have a garage sale to get rid of excess stuff before putting your home on the market.
Most importantly (and simply): Clean sells! :)