I hope everyone is having a great weekend, and for those in the U.S., I hope you do lots of fun fireworks or something to celebrate the 4th! I myself am having a grand time visiting my in-laws in Eastern Oregon - I sneakily (or not so sneakily) wrote this post a few days ago and post-dated it. I love technology :)
Here's a little idea I have (but have not tried yet) - why not have a sight reading competition in your studio?
You could challenge each student to keep track of their sight reading - either by how much time they have spent or how many lines or pieces they have sight read each week. Keep track on a big, colorful chart in your studio and at the end of the month (or however long you decide the competition will run), the winner gets a prize. If you teach a lot of advanced students, you could challenge them to sight read pieces from the piano literature of the great composers (and maybe get in on the fun yourself!)
I think this would be a wonderful way to motivate your students to sight read, and to get them to make sight reading a habit.