Plan ahead for items like textures you might want to emulate. If you want a marble-looking surface, you'll need to have the "stone" as smooth as possible, before any painting occurs. In keeping with the last statement: attempt to create a single, homogenous surface so that any paint you apply adheres the same overall... this is true for smooth, rough, or any kind of suface. For a more weather-worn appearance, you may need to distress the lettering (field) surface BEFORE you apply trim pieces (that would otherwise be in the way of your tools).
Even though I've stressed careful planning and workmanship, don't worry EXCESSIVELY about minute details. Typically, it'll be dark and your viewers won't see those twenty layers of subtle shading you so diligently painted on your masterpiece. Moss effects may be a nice touch, but at night, under blue or red lights, who's going to see that quarter-sized blob of faded green, that for two hours you so painstakingly applied.
However, where necessary, correct as many obvious imperfections as possible in the light of day... especially before painting takes place. Be sure to fill any holes, dents or cracks that shouldn't be there... but don't worry about being too fine at this point. Remember, it's easier to knock down a bump, or ridge, of joint compound once it's dry than try to make it a perfectly smooth surface. So, go ahead and slop on what's satisfies for the final dimension... or even, just a little bit more. Nothing worse than leaving yourself short, and then having to apply yet another layer.
So... have you noticed a theme here? I think it's: plan... plan... plan... and plan some more. Know what you're GOING to do (before you do it) and then execute it. In almost every discipline (from delivering a karate blow... to a brushstroke onto watercolor... to baking a devil's food cake) one thinks through the act before going into action. Without planning, the act becomes random and meaningless. And in this case, randomness will stop you cold and probably ruin the project.