Well, I'll throw in my two cents too.
For lighter-weight stuff, you can go with lighter shelving type.
For up high, just a foot and a half or so from the ceiling, I like to use those wire closet shelf systems from the big box stores. They're the white ones.
If you space the brackets every foot, and really anchor them into the wall, the load will be distributed pretty well. And like you said, you won't be storing cylinder heads or other really heavy things up there.
Besides being readily available, fairly innexpensive and really easy to put up, the other BIG benefit I like them for is that they let the light through. Typically, with shelves on the wall, you're always going to have a shadowed area. Solid shelves make a garage seem darker. The wire shelves let light through, which eliminates the dark areas on the wall.
That may be a little thing to most people, but you'd be surprised. It also discourages spiders from building webs, which means you won't have as many cob webs either. Plus, they don't catch dust like regular shelves, so your stuff stays a little cleaner (you aren't pulling stuff off the shelves through a layer of dust).
I stacked A LOT of stuff on those shelves, including '40 Ford backing plates, air cleaner assemblies, exhaust manifolds, spray cans of paint, a radio, boxes of misc. parts, a couple aluminum intakes, and that's just what I can remember off the top of my head.
The key is to be very generous with the number of supports. Mine was on a concrete wall, and I used the self-tapping Tap-Cons every foot. Lots of drilling, but it worked out really well.
-Brad
I'll post a pic of them later on today once I make my way into the dungeon I really need to fix my light's 
