If you use a tablet and stylus, you can get some pretty good results for straight lines and curves using ordinary rulers and curves. The color image I did today at the studio on my Cintiq (which granted made things a snap for lining up what I was doing visually). The curves were done at home on my Wacom Intuos 2 using a plastic ruler, compass ruler, and curves. I decided today to make myself a tool for the studio for straight lines. I bought an ordinary clear plastic ruler, some double stick tape adhesive, and some felt. I attached two thin strips of felt to the under side of the ruler near the edges. This not only protects the screen but buffs dust off as well while I use it. It makes creating straight lines a snap on any screen paintable hardware you may have access to. Tablet PC users could make use of this.