I NEVER start with a slip knot because I don't want to have to pick it out. Just hold your two ends on the back of the needle and start your cast on.

I always cast on with two ends (both from the same ball OR two balls.)

In the picture I have used two different colors to do the Twisted German Cast On. Because I am working TOWARD the balls, I can cast on an infinite number of stitches without any tedious measuring of my "long" tail.

When I am finished casting on, I cut off the end I do not need.