The factory edge on a fresh sheet of plywood is very straight. So use that.
Option 0: Snap a chalk line and cut by hand. Fine if you're doing sheathing, but not accurate enough for many other tasks.
Option 1: Use 2 sheets of plywood. Stack one on the other, offset by ~32". Use the top one as a guide to cut the bottom one.
Option 2: Make an 8' ripping guide.
You'll need the factory edge of a sheet of plywood, the full 8' long. Approximately 6" wide. Thicker is good. This will be the guide.
Also, a piece of doorskin (single-ply, 1/8" plywood) or 1/4" plywood, 8' long, approx 8" wide.
Glue & screw them together. Make sure to countersink the screws slightly.
Run your circular saw along the factory edge of the thicker piece, to cut the thin lower piece to match your saw.
Now you can put the fresh-cut edge on your material and know exactly where it will cut.
This jig is specific to the saw you used to make it.
There are many videos covering this on YouTube. Here's one: http://www.youtube.com/watch?v=mwcOKX7PaRg
For bonus points, make it double sided (one narrow, one wide), so you can cut on either side of your circular saw.
Option 3: A guide that attaches to the saw, like the Kreg Rip-Cut: http://www.kregtool.com/RipCut-Prodview.html
Option 4: Festool Track Saw http://www.festoolusa.com/power-tools/track-saws/ These circular saws fit on special tracks to make straight cuts. Not only do they cut through heavy, thick wood with ease, but they cut through wallets even faster!