Okay here we go..
I used this one on my felted clogs. It worked well. Info says it's washable which would be great on knitted slippers. My DH put this on, but if I do it again I'll make a pattern to hold on them. Just to make it prettier. It dries clear though.
http://www.amazon.com/Performix-0758...rip+for+fabric
I haven't tried these, but they look cool. It says they are washable, too.
http://www.grippiesonline.com/home.html