The single most common error students make in writing resonance structures and reaction mechanisms is incorrect use of the curly arrows that are employed to keep track of bonds made and broken. Curly arrows indicate electron flow, NOT movement of atoms.
Thus, the tail of every curly arrow should lie against a pair of electrons that is to be moved, and the head should point carefully and directly to the new location for those electrons. Only three distinct actions are diagrammed by the curly arrows:
Here are three examples of how - and how not - to use curly arrows.
In the first incorrect case, the curly arrow is moving a proton instead of the electrons that will bond the proton. Similarly, in the second example, the methyl group is being moved by one of the arrows, instead of the sulfur electrons that will form the bond to it. The third incorrect case has everything wrong, including an arrow that seems to have no purpose, since no bonding interaction between O and Br should be generated in this reaction.
Using the curly arrows correctly will make a big difference in how easily one can generate a valid mechanism.