Using correct punctuation in dialog is essential, yet it is one thing I see done incorrectly as much as anything. For example, when someone asks a question, be sure to punctuate with a “?” I can’t begin to tell you how many times I’ve seen dialog that was clearly a question, yet didn’t employ a question mark.
Admittedly, there are times when this is not 100% clear. “He wondered whether the police had all the evidence” is a statement, but “Did the police have all the evidence?” is a question. One way to figure it out, if you’re in doubt, is to say it out loud.