Feeding with both breast and bottle allows your little one to get all the immunity of breastmilk, while you gain a little independence. It also gives dads a chance to be more involved. If you mix-feed from the start, make the transition as soon as breastfeeding is established and before your baby is 6 weeks old. That way, she will not be so used to breastfeeding that she finds a bottle strange.
If you want to do it later on, say, when you're going back to work, start mix-feeding about three weeks before, to establish a routine.
Just breastfeed your baby at the feeds you want to continue with - on waking and at bedtimes usually - and bottlefeed the rest of the time. If your baby rejects the bottle, ask someone else to do it.
Also, experiment with different bottle teats, and offer expressed breastmilk instead of formula. If your baby is about 6 months old, you could try a first cup.