I have 1x12 shelving in my pantry area in which one wall is about 7'. Each board on the longer length is braced with three of what are sold as 'utility shelf brackets' in most hardware stores. They're not as fancy as the adjustable tracks that Tester101 referenced, but they tend to sell for about $1 each in the stores, and they allow the board to go fully against the back wall (as there's no upright in the way), making it easier to put a strip along the back to deal with sagging along the length of you ever have to. You also screw or nail the bracket into the shelf, so sagging is greatly reduced compared to just setting a board across a floating support.
The only issue with fixed shelves is that you have to know what sizes of things you're going to be storing. Mine are of differing heights; the bottom could hold a 2L bottle with an inch to spare (although, I keep them on their side in little cubbies, sized so it'll 64oz juice bottles or a double-wide stack of boxes of chicken stock). The others can hold a double-stack of 28oz cans with a couple inches to spare. I keep larger stuff up above (eg, large cereal boxes, rolls of paper towels), as those tend to be lighter things.
The only problem things are really small jars, where you're tempted to stack 'em 3 high, but I'm afraid of them falling over. That might not be a concern for most people, but my pantry space is along my basement steps, so I'm always afraid I'll hit 'em when carrying the laundry up & down ... other people likely won't have that concern ... but I still find 2 high means I don't have to dig through stuff to retrieve what I want.
(Note: except for adding the cubbies and some other modifications to extend the bottom sill, this was all installed by a previous owner and they're painted, so I'm not sure what type of wood it is. To the best of my knowledge, the utility brackets and being attached to the shelf on the short length are the only support used)