Here's a simple but effective way to hang a big poster - thin strips of oak pinning the print at the top and bottom, and suspended by a copper wire. They're inexpensive and easy to make and customize.
Step 1: Design
These rails are imitations of Parabo Press's wooden poster rails ($59). Instructables recently ran a promotion where they gave away their 4'×3' Engineer Prints (just pay shipping). This was a great deal and my wife and I both got one. Mine was in portrait and hers in landscape, so even though we liked their wood poster rails, they only come in 36" and we needed 36" and 48". So I made some instead. Luckily, the Parabo app has some nice closeup pictures so I could duplicate them fairly accurately, though I used screws instead of magnets. Because I had a four foot length of 3/4" × 4" oak, 1/2" #4 screws, wood finish, a picture hanger and copper wire in my workshop already I was able to make two sets of these for nothing (if you had to buy the parts, you'd be looking at ~$20 but you could make multiple sets of rails).
Step 2: Cut Wood
Rip the 3/4" (18 mm) oak plank into 1/4" (7 mm) strips, then trim to length: 1213 mm if hanging in landscape, 913 mm if in portrait. If you don't have a table saw, fret not; just purchase some similarly-sized trim at the local home store. It will be pricier but will come with a nice level of finish that won't require any planing or sanding. I had a nice new blade in my tablesaw so my strips only needed a little cleanup with a sanding block.
Note that I actually got 9 strips out of the 4" width of oak; this was deliberate in case one broke or I made a mistake. You could make each strip a hair thicker if you like.
Step 3: Pre-drill Screw Holes
Now - and this is important! - match up your strips so any bends are opposed, so they cancel each other out and your strips don't warp. Line the pieces up perfectly, clamp, and drill holes near each end and every ~200 mm along the strip, drilling all the way through one piece and halfway through the other. Drill wider holes in the back piece and countersink them for flathead #4 1/2" screws. Drill a couple of vertical holes above the holes ~200 mm from each side. This is where your wire is going to go.
Step 4: Finish
Step 5: Frame and Hang
"Framing" the posters was easy - I just lined up the oak strips and screwed them together from the back, straight through the print. When doing the top rails, I used a piece of copper wire 3' long and fed it through the holes as shown, then added screws to trap the wire (this would work just as well with string). Hang in a location of your choice. We're very happy with the finished product, and ended up with 24 square feet of inexpensive personalized art.
If you want poster hangers that do not puncture the artwork, try drilling shallow holes for rare-earth disc magnets using a Forstner bit and epoxying them in place. Use washers on the other side. This worked well for me in holding the stickers on the Rubik's cube chest of drawers I built.