Lacrosse balls are okay, but often they are too hard for me and I have to be extra careful to get the pressure just right. Tennis balls are often too big to dig into the spots that I want to work. I like the yoga therapy balls from Jill Miller. They are smaller than tennis balls and firm on the inside, but with a soft wrapping. They also have a little tote that makes it easy to sling them over the back (not that a sock wouldn't work too).