What points should be kept in mind while drawing resonance structures?

(N/A) $(i)$ The positions of atomic nuclei must remain fixed in all resonance structures.
$(ii)$ Only the positions of $p$ or $\pi$ electrons should be shifted.
$(iii)$ If possible,place electron pairs on more electronegative atoms to make them negative,and ensure that the number of unpaired electrons remains the same in all structures.