I haven't found one technique that works on everything. I favor the marking pen in neon blue that washes out. For black and very dark colors I use white chalk, but that leaves a residue that doesn't always come off. Recently saw a marking pen that comes off when ironed. The pencil marks have always come off for me when I've used them but drawing a straight line on flexible fabric is hard for me.
Have fun playing around and good luck finding a technique you like.