Writing Resonance Structures

Position of nuclei must be the same in all structures, otherwise they would be isomers with real existence.
Total number of electrons and thus total charge must be constant.
When separating charge (giving rise to ions), usually structures where negative charges are on less electronegative elements have little contribution, but this may not be true if additional bonds are gained.
Resonance hybrids can not be made to have lower energy than the actual molecules.