How do I stop my cat from using my bed as a litter box?

I have a 9 month old cat who is using my bed as a litter box, both 1 and 2. She did this awhile ago, but stopped. But now is doing it again, even though I keep her litter box clean, and use the same kitty litter. When I am home and she does it, I put her nose in it and put her in the litter box, which she hurriedly jumps out of. It is getting so frustrating to come home from work / school and have to deal with cleaning sheets and the mattress. Is there anything I can put on my bed that would deter her from doing it?