Uh, I keep em. LOL, that's the short version. Usually, lately, I have just been getting one 'contributor's copy' from the magazine, then I also subscribe to the magazines, so I get that copy. I subscribe because it supports the magazine/industry, because if I'm in it it's another copy I get, and if I'm not in it, then I can see what is!
The first copy I receive is usually a little worn from me looking at it so much, so that one goes on my book shelf, eventually. The next one that arrives, I keep crisp and I put it in my "portfolio", which is silly, because it is getting heavy with all the magazines to carry around them all! If I get a third copy, I tear out the page(s) that have my design, this is called a 'tear sheet', and I try to remember to send this with a short note to the yarn company who supplied the yarn for that particular project. Not all the designers do this. One of the magazines actually provides an extra, loose, "tear sheet" when they send the contributor's copy. Also, with the tear sheet, you can put that in a portfolio instead of the whole magazine. Portfolio's are only really necessary (IMHO) if you are going to see a buyer in person, like at a conference. I have a 'resume file and a portfolio handout' on the computer that I can e-mail, I ought to tell you about that one day too.
This is a photo of Charlie chasing a mosquito that got in the house. He jumps up as if he is going to catch it, crazy cat.