First of all I just want to say that breast is best, but unforunately after breastfeeing for 2 months I was only able to feed on one side my baby was not getting near enough to eat and was colicy until we switched her to formula. At first I felt like a failure, but don't feel this way! Our baby is so much better on formula. So if you have to switch here are a few suggestions.
1. Pick the right formula. If your baby has milk intolerance or is a premie you may want to look into a formula that is designed with her special needs in mind. I love similac formula. Our baby does great on their product.
2. Pick a bottle that is like breast when switching from breastfeeding. Once you find a bottle you and your baby like buy enough so that  you are not stressing over washing bottles all the time. I like soothie bottles, but it is up to you. The nipples are fat and to me resemble the breast a lot.
3. Pick the feeding baby is least interested in and feed her a little early, say 30 minutes. If baby is starving and frantic she will not take the bottle as easy.
4. Let someone else feed baby the bottle. If you do it, baby may smell your milk and not take it as easily. I always even left the room when my husband feed her from a bottle at first.
5. Start with one feeding (for your babies sake and your own), and work your way up. You might start with substituting one bottle a day for a few days and then two bottles for a few days and so on and so forth. That way your baby has time to get used to the idea and so do your breasts. If you go cold turkey baby can be cranky and your breasts will be sore.
6. If you are going back to work and are substituting formula be sure to give baby the bottle for the same feedings she will be missing when you are at work.
7. And finally be patient. This is a process it may take time. Remember, this is new to baby and make take some getting used to. Good luck!!