There's not much information about it here, and I'm no expert; but one thing that comes to mind is whether or not your sign looks professional. There are a couple of restaurants that have shown up in my area, and my friend and I so often have said how their sign makes a person not really feel very confident about going in there. They look "rinky dink". So often, these places just don't last long. Right now there's a pizza place/other-food place that has a sign that looks like it's made of painted wood, and the lettering looks like someone probably used a stencil. It just doesn't look polished.
We were saying how we don't necessarily think a small place needs a giant, expensive, sign; but it seems like the places with something more professional looking do stay in business longer (at least where I live). (Again, I'm no expert; and this is just one person's - well, two people's) opinion.)