That's a good question. Expressing does increase supply, but your baby feeding increases it more. When supply is low, feeding more frequently is a better option than expressing between feeds, although both should help.

In the first 6 weeks it's important to get the "demand" set up so that the supply gets "set" to the right level. Usually it's not a good idea to express regularly during the early weeks as it can lead to an oversupply (which believe it or not can be a problem also). If you are feeling full and uncomfortable after a feed, it is best to express enough off so that you are more comfortable to reduce the risk of mastitis. Expressing until you are empty though is sending a signal to your breasts that your baby needs more milk, so isn't such a good idea. After the first 6 - 8 weeks, if you feel up to it, it is fine to start expressing to build up a store of EBM. But only if you want to!