I've never been able to get a tennis ball to work for me, but I do use a wooden kitchen rolling pin on my legs. It really helps work out the knots, and I can do it myself.
There are also products like the Theracane and Body Back Buddy (which I have) that are shaped like canes with knobs on them that you can use to work out trigger points. Lots of online stores sell them, but I think I bought mine at Amazon. It's become invaluable to me to work out knots on my back when no one else is around to help or they've just become immune to my pleas for a massage.
Hugs - Austen