If you need to bottle feed, either expressed breastmilk or formula, choose a slow flow nipple. You want to slow the flow of the bottle down so the baby will have to put some effort into feeding. Then he/she will be willing to do the same on the breast.
Bottle feeding should take approximately 10 minutes for one ounce, 15 to 20 minutes for two ounces, and over 20 minutes for three or more ounces. If bottle feeding is taking over 40-45 minutes, perhaps you need a faster flow nipple.
Make sure to bottle feed the baby in a fairly upright position (70 degree angle), with the bottle fairly horizontal. Baby has to work a little more this way. It also decreased the amount of air the baby gets. Allow the baby to take a break and burp after each ounce.If liquid is dripping from the corners of the mouth or from lower lip area, the flow may be to fast for the baby.
Before buying new nipples, compare the ones you already have. Put water in the nipples and turn them upside down in the sink and watch them drip out. Use the slowest one you have. If feeding still goes too quickly (see above) or fluid leaks from the baby's mouth, buy a slower flow nipple.
When buying a bottle/nipple system, understand that the nipples are not standardized. Not all "stage one" nipples have the same flow. Even two of the same brand and stage may have different flows.
Make sure to buy a round nipple, no flattened or orthodontic nipples. These can change the way your baby will suck on the breast.
In general, the nipples given to you by the hospital (latex or red) tend to be very fast flow. Also, Born Free and Avent tend to be faster that others. Don't throw them out, you can use these later when the baby is older. Playtex drop-ins and Dr. Brown's tend to be a little slower. The slowest flow bottle may be the special needs feeder by Medela and the Adere.