I burn through a ton of grains brewing 10 gallons a week. I always have 55lb sacks of American 2-row, Pilsner, and Pale Ale Malt (english base malt). Most people can burn through these in a few months, but if you store malt properly it'll last a year or more. I brew enough that I also have honey, vienna, munich on hand in 55lb bags.
I did buy a 55lb bag of chocolate and caramel 60. That was a mistake. I haven't used hardly any of those yet. I'm sure they'll go stale before I use them. I give that stuff to anyone that wants it.
In short buy the sacks of your base malts, maybe some higher volume specialty malts like munich, but never buy sacks of black patent or anything like that.
As for how to go about doing this, I live close to a malter so I just drive down there and he loads me up with whatever I want. Not many of these guys around anymore with the big malting companies buying everyone up. Your local brew pub might consider tossing a couple extra sacks on their pallet if you ask nicely. Otherwise you need to go through one of the big malting companies and you'll probably need to pay for a pallet and delivery. If you live close to a warehouse you may be able to avoid this. Homebrew shops also will sell you bags of malt sometimes.
Buy all your specialty malts at your homebrew shop though, and support those LHBS.