One way is to use the paths tool. (I use GIMP, hopefully this works for other programs too).
Make a straight line with the path tool. Click on the path part way along and pull it to one side, you can adjust the angle of the curve until you're happy. If you want an S curve, repeat the above.
When you're happy with the line, "stroke" the path with the desired colour/thickness. If you need lots of lines, you can cheat by copying the original on to new layers and then stretching/morphing/recolouring them for variety.