I have a great chair that I use for practicing, but it is upholstered with faux leather (i.e. plastic), and it can get uncomfortably hot and sticky after a while. My practically cost-free solution? Take two old T-shirts, and some straight pins, and follow me! It will take you ten minutes at the most to do.
I used two worn-out cotton T-shirts, one for the seat, and one for the back. If you slide the arms of the seat shirt through the legs of the chair, it should stay in place. A rubber band or some kind of tying device of your choice on each leg will keep it snug.
Turn the shirt inside-out, and slide it over the back of the chair.
Starting from the edges and working towards the middle, pin the hole that is meant for your head together.
Now pin the arm holes shut.
This is hard to see in this photo, but the last step is to pin the back seam all the way down.
Turn the shirt rightside-out, and slip it back over the chair. Here is how it looks before sewing, which is pretty much the same way it will look after sewing (which you don't even have to do!). All the pins are on the inside of the shirt and on the back of the chair, so there is no chance of getting stuck.
Have a seat!