Get the cork-backed ruler

The cork-backed ruler is the answer if you want a straight line without the ruler sliding around.

Cork grips paper like nothing else. It’s not fancy — it’s just a thin sheet of cork glued to the bottom of a metal ruler — but the friction is real. You can buy one at any art supply store for a few bucks. Westcott and Staedtler both make good ones.

If you already own a ruler you like, the masking tape trick works fine: stick two short strips of tape to the underside, one near each end. Tape grabs the paper hard enough to stop the slide. Just don’t cover the edge you’re drawing along.

One more thing: if you’re using ink, get a ruler with a beveled edge that lifts the ruler slightly off the paper.