jessicasmummy - I was concerned about this as well, as ds was on mixed feeding from about 10 days, because of excessive weight loss, not putting it back on and getting really sleepy. I think part of it was just his nature - he didn't take enough milk to stimulate a good supply, so there wasn't very much milk for him, so he got sleepier. He still isn't overly interested in food.
BUT...with dd I was DETERMINED to make it work, if only to prove I could. I felt a real failure with ds, even though I ended up feeding him till 13 months, I didn't manage to exclusively breast feed him at all. Dd is a different character - very motivated by food! But I had a few rules I set myself - feed every 3 hours minimum, day and night for the first few days to really get the supply going - this sounds militant, but it really worked. I also resisted going onto bottles, even though she lost lots of weight as well - having had ds who did that, they were more relaxed about it (they were both huge babies , but smallish children, so I think they were finding their own level). Keep an eye on the nappies as they are the best guideline as to how they're doing - don't worry if they don't poo very often, but in the first few days it should start to go yellow. Wet nappies are very important. Don't assume that if the baby cries and won't settle that it is hungry!
Good luck - I managed to exclusively breast feed dd for 6 months, before introducing solids, and fed up to 14 months. She didn't have ANY formula at all, so it can be done. I don't think I had a huge amount of milk with her either, but was more confident in what I was doing.