I just keep my fingertips lightly pressing on the side it is not being brushed. To avoid fingertip marks on the film use latex gloves. In my case I use the glove in my left hand and brush with my right.
You might also want to use less developer. I dont know if it is possible with the developer you are using but I use 500 ml for an 8x10 neg on an 8x10 tray.