Make the final arrangements the day before. If you were preparing for an important interview or hosting a party, you'd make sure to handle the details of cleaning, shopping, selecting outfits, etc. in advance of the big day. Your birthday is a big day too, and your goal is to make it as special and relaxing as possible.
Tidy up your home in the day or two before your birthday. For most people, it's hard to relax in a mess, and you want your home, especially for your solo birthday celebration, to be an oasis.
Make the place look festive: you can go all out with streamers and balloons, or just brighten up the space with a small bunch of fresh flowers (another splurge item you might not buy yourself regularly) or candles.
Choose your birthday outfit the night before: pick something that's comfortable and makes you feel awesome about yourself.