Rules for Resonance

Resonance forms are imaginary. The real structure is a composite (hybrid) of the forms we imagine.
Don't move nuclei (only electrons "move").
Octet rule should not be violated (no more than 4 electron pairs).
Formal charge follows normal rules. Hint: the total formal charge will always be the same.
The more contributing structures, the more stable (but contributing forms need not be equally stable).
The more stable a contributing structure is, the more it contributes to the overall picture. The less stable it is, the less it contributes.

How to Evaluate Contributing Structures

Stabilizing influences

Examples

More bonds; more atoms with octets

The structure on the left is more stable than that on the right partly because of the increased bonding, and partly because all of the atoms have octets. Note the two radical carbons in the structure on the right. Radicals have unpaired electrons (seven in the valence shell of carbon here). In this case, the formal charge of each carbon is still zero. The funny curly arrows indicate the formal movement of one electron for each arrow.

Keep unlike charges close, keep like charges apart

This pair of resonance forms looks like the one above. However, instead of moving electrons singly, we move them in pairs, as indicated by the double-headed arrows. The form on the right ends up with one carbon bearing a non-bonded pair of electrons (and a negative formal charge), while one carbon has only the three bonding pairs, for 6 total, and a positive formal charge.

Neither of the resonance forms on the right are very stable. Nonetheless, they contribute a little bit to the structure of conjugated dienes, as we will see.

Keep bond lengths normal

A normal bond length is about 150 pm, give or take a little. Remember that you can't move atoms when drawing resonance forms! The resonance form on the right doesn't contribute because the bond that would be formed is much too long.

Place negative charge on more electronegative elements; positive charge on less electronegative elements

You can move the pair of pi-bonding electrons of a double bond (a common resonance move), but you will have to choose which way to move them. In this case, looking at the charges will help you--put the negative charge on the more electronegative oxygen, and the positive charge on the less electronegative carbon.

Stabilizing influences	Examples
More bonds; more atoms with octets	The structure on the left is more stable than that on the right partly because of the increased bonding, and partly because all of the atoms have octets. Note the two radical carbons in the structure on the right. Radicals have unpaired electrons (seven in the valence shell of carbon here). In this case, the formal charge of each carbon is still zero. The funny curly arrows indicate the formal movement of one electron for each arrow.
Keep unlike charges close, keep like charges apart	This pair of resonance forms looks like the one above. However, instead of moving electrons singly, we move them in pairs, as indicated by the double-headed arrows. The form on the right ends up with one carbon bearing a non-bonded pair of electrons (and a negative formal charge), while one carbon has only the three bonding pairs, for 6 total, and a positive formal charge. Neither of the resonance forms on the right are very stable. Nonetheless, they contribute a little bit to the structure of conjugated dienes, as we will see.
Keep bond lengths normal	A normal bond length is about 150 pm, give or take a little. Remember that you can't move atoms when drawing resonance forms! The resonance form on the right doesn't contribute because the bond that would be formed is much too long.
Place negative charge on more electronegative elements; positive charge on less electronegative elements	You can move the pair of pi-bonding electrons of a double bond (a common resonance move), but you will have to choose which way to move them. In this case, looking at the charges will help you--put the negative charge on the more electronegative oxygen, and the positive charge on the less electronegative carbon.