For straight lines I use masking tape! for some reason I just don't trust chalk, I know many people do but with the tape if for some reason (I have three kids so thats reason enough) if I have to stop in the middle of a line I don't have to worry about the chalk getting brushed off while I am away! the tape stays in place pretty much no matter what, I had some tape on a quilt until just the other day, it had been on for at least three months folded up in a box, i pulled it out put it in the machine and stitched the line! removed the tape and nothing was left behind and the tape had stayed in position!
You can also get tape in many different widths to get different width lines!