There are many ways to get students to slow down and truly think through the artistic question at hand. Almost every project I design includes a little pre-writing, and sketching. This way work is more focused and there is less waste of supplies. Here's a sample of one below.
The results though are counterintuitive... when students begin to understand that there is always more work to do, they slow down. They take more time on the main exploration, and rush less. After a few projects, fewer and fewer need to make use of the "I'm Done" bin.
The bonus is that when I need to be out for a day, the sub can grab any assignment out of the bin, and copy it for my classes, or allow students to choose their own exploration for the day. My policy is to always grade sub work so students know it must be well done.
These collections were fairly easy to make. Open a Word Document and put a sketch prompt on the bottom of each page. When you print, the students draw right above the prompt directly on the print-out or photocopy. This way you're not wasting any sketchpaper. You can do the same by putting quotes in the middle of pages leaving lots of white-space for students to illustrate the meaning of the art quote.
This has been such a lifesaver for my program (and sanity) that I have made collections of these "I'm Done" assignments and published them into books. I encourage teachers to cut the binding of the books and copy the work for their own "I'm Done" bins. The books can be found HERE for 30% off Amazon's price. But again, you can make these fairly easily on your own if you have the spare time.