Require your students to use a couple of book resources when giving them assigned papers. Instead of just saying, make a research of this and that, be specific of the book title, authors, and all other related textbooks that they can use as a reference for their assigned paper. Most students has the tendency to search over the net, then print a copy, and that's what we're trying to avoid.
Encourage your students to proofread their papers and create a draft, cause more likely you'll end up putting red marks on their works, to prevent wasting ink/toner and printer paper. It would also be better if they send their paper works on your email so you can check all the mistakes and necessary remarks before it gets printed. If it's not a good idea on your part, then tell them to use the draft settings of their printer to use less ink/toner.
Remind them the use of Print Preview, some students are not fond of using this but believe me it really helps a lot on misprints, especially when it comes to page numbering, there are instances that a page number doesn't appear to a certain page, and you have no other choice but to reprint it.
Black prints only, can also be implemented. If the use of color inks can be avoided then do it. Students usually add unnecessary borders, lines and images probably to reach the minimum page limit. They should know that the content of their work matters most and not the design. For photos, if it really requires to print some, then you may suggest using gray-scaled settings when printing images.
Make them understands, that quality is always better than quantity. Longer paper works, doesn't always get better grades. Sometimes the shortest term paper that is full of substance, is way better than longer ones. Just like buying printer consumables, quality is not only measured by the price, but by its performance. As a teacher you should practice what you preach, and that you're using eco-friendly toner cartridges in your house or even at the faculty room.