I do what Tyler suggested. I can't draw or see a straight line to save my life. My vinyl has grids on the back and I try to cut it so that my I always cut it almost straight that way my perception of feeding the vinyl into the machine isn't skewed.
One vinyl roll doesn't have the grid so I am getting better of eyeballing it to where I don't cut it too crooked. But running it back and forth and looking at the measuring grid on the machine helps alot to see if it is drifting to the left or right.
I bought a media tray from Signwarehouse that I use to put my roll of vinyl on and it will roll while feeding the vinly into the machine.
http://www.signwarehouse.com/p-SE-ES-ENDURATRAY.html There is probably something like that you could rig up, but I'm not creative into making up something so bought this instead.