If you are breastfeeding, it is true that it is pretty handy to have some back up milk in the freezer.
If you ever plan to be away from your baby for more than a few hours (let me just say that if you don't plan to get away some, you will likely go insane, so just plan on it now), someone else will have to feed your kid. And if/when you are going to back to work, get sick, going on vacation, any of these things can disrupt your schedule (and your eating/hydration schedule, too) and might cause your milk supply to dip, so its great to have some backup on hand, of for nothing else, then peace of mind. Not to mention if your baby hits a growth spurt and you haven't enough fresh milk on hand to keep up. Our little guy hit a spurt right around 3 months, which is when he started at daycare and I started back to work. Trifecta for us. I took some pressure off myself because I had a pretty good supply in the freezer for just this case.
OK, OK, you say. So I'll get to work on my freezer stock... but how? I wondered this myself. When I was about 6 or 7 months pregnant, I recall asking one of my new-momma friends how she ever "got ahead" to be able to build up a freezer stock of milk. She thought back to her first "daze" on the job and said, "you know, I don't remember". So I made a conscious effort to pay attention. At first, you do not work on this supply. Depending on how well breastfeeding is going for you, you just work on that, and bonding with your baby. Once you start to get the hang of things (maybe a couple weeks? You'll know because you'll have a split second to think: wow, a free moment - what should I do? If you even think you might be tired, though, just take a nap instead.), I suggest pumping sometime after the first feeding of the day, maybe when the little one goes down for their first nap, maybe for 15 minutes or so. There always seems to be extra milk in the morning. And I would wager that this is a good strategy to up your supply in general. Lactation is all about supply and demand, so keep demanding. The supply built up because until the baby was 3 months old, I wasn't away from him much at mealtimes, and I was getting an extra few ounces of milk nearly every day.
I read in a couple of sources that folks recommended freezing milk in one or two ounce portions, so that this was more flexible for you to use. But I just found this wasteful. Once you thaw one of those little baggies, there is all kinds of milk still clinging to the sides, so why do this 2 or 3 times just to get a full bottle? By the time I was pumping to freeze, my guy was drinking 3-4 ounces per feeding anyway, so I reasoned that there was no reason to freeze any less than that. Now that he can down an 8 or 9 oz bottle, I only freeze in 6 oz portions. In both cases would just top off whatever I'd thawed with refrigerated milk, and voila, a full bottle is good to go.