I have lots on the wall, I used that gummy blue stuff that comes in small packages. I think the shinier your wall, the harder it is for stuff to stick, but that blue stuff works great for me. I also have drilled small holes in the cd's and strung them up, so they hang vertically. It required several drill bits and alot of cracked CD's but I was able to do it. I did have someone donate a box of 500 CD's, from a flawed general ledger run at a local business, so I had lots of CD's to work with. Many CD-type ornaments on the tree at church, too!

I stole the idea from a UMC church in Minnesota.
They used blue tacky stuff like Barb did.
I however, experimented with some 3M tape products. They have many different kinds of double sided squares. The best ones we found were about 1/2 inch square clear and had some thickness to them, rather than just thin double-sided tape.

Occasionally a CD falls off, and I know the tape can damage the paint a bit in a very few spots, but we're planning on repainting someday anyway.

I did a pyramid because I got tired of putting them on the wall -which I originally intended to completely cover.

I had a lot of "sticking time" to think about other options. One idea I had involved hot-gluing CDs to bands of colorful cloth that could be hung and taken down if necessary. Cloth would also aid in reducing some sound reverb in the room. I would fasten edge of cloth to pvc pipe and hang the pipe on 3 nails. Could do the same thing across the ceiling (for the daring), or glue them to 4" strips that hang.

I'm sure there are plenty of ideas others far more creative than I will come up with. <>< Neil