A supplemental nursing system, which are available from all the brands which also sell breast pumps, is a great way of stimulating your milk supply while supplementing your baby's milk intake and keeping breastfeeding going.
Most mothers mention that it's difficult to get used to initially, but once they've got the insertion of the tube going well (and of course the latch on of their baby) everything settles into an easy rhythm.
Here are 3 videos I've found which really show you what it looks like to use an SNS.
You can see that there are a few different ways of doing it, either inserting the tube after the breastfeed has begun or placing it at the nipple before latching on your baby.
You'll know it's working because you'll be able to see the milk flowing up the tube.
I particularly like the suggestion of keeping the bottle between your breasts to maintain the temperature of the milk (and remember you can use breast milk or formula in these tubes - I think that formula can be a bit thick and difficult to clean out at times, but there are some great, supportive videos on YouTube that help you with this problem - if you have access to a milk bank or donor milk though, go for it!)
I'm a fan of having the tube pinched off at the beginning of the feed to help you assess how much milk you have. Once your baby slows down with their breastfeeding and breast compressions don't help to increase the milk flow then open the tube up for the extra milk.
So really, a supplemental nursing system is just as easy to use, once you've got used to how to get that tube in, as supplementing with a bottle. You're still sterlising just as much, but you get the added bonus of the closeness of breastfeeding, your milk supply being stimulated, your baby learning to ue their tongue and lips in a way that helps develop their teeth spacing well and the wonderful relaxing hormones that breastfeeding releases in you to help you cope with the crazy newborn time!
Another page you may be interested in reading is the Hypoplastic Breasts page, which describes why some women can't produce enough breast milk.