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One of my favorite ways to do that is by “publishing” books for them. This can be as simple and inexpensive as taking their writing and art and cutting and pasting it into a cute journal from the dollar store. Or just giving them a small, blank scrapbook to create a book. I keep a whole box of these, ready for the taking. There's not a book in here that cost more than $1.
Also in this box are inexpensive photo albums that they can slide their individual book pages into, easily making a hardbound book of their own. 
Here is a book that we made just by folding some cardstock, punching two holes in the center, and tying yarn through it to bind it. So simple
Sometimes you can catch photo websites such as shutterfly.com and snapfish.com when they are having a free book deal. I used one of these opportunities to publish a book Riley had worked really hard writing. All I had to pay was shipping.
This was fun also because I could add stuff like a "Meet the Author" page.
I also try to treat their art very seriously. If it is something really special to them, I like to frame and display it. That doesn't happen as often as they would probably like, but I try. Here are a few of ours.
Here is my new favorite. Cole made these pictures for me. He drew my favorite coffee cups. I love this so much.
I want them to know I think their art is worth a lot to me. Plus, it’s adorable! Be on the lookout at garage sales and dollar stores for frames for this, too.
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Something about having your story in book form, or your painting on the wall, makes you feel like your work was worth the trouble and makes you eager to do more.
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It can be cheap and doesn't take much time. It's worth the extra effort!