I used clean-release hooks - these are called decorating clips and they are very small. You can see how I've used them in the past to hang my necklaces here.

They are almost invisible on the wall.

I put safety pins on either end of the flag (at back along the top) and one in the middle.

I pinned them so that they don't show through to the front.

Then I clipped the safety-pins onto the decorating clips.

Ta-da! Our flag is hung. (And I might even take it down and iron it one day. Who knows!)
