When writing dialogue and dialogue tags, I have always asked myself if the tag is a complete sentence. If so, I use a comma and lower case such as:
She said, "Bob." "Bob," she said.
"She said" isn't a full sentence on its own (in this context), so I use a comma instead of a period. I know this is correct. However, when it has either an ! or a ?, such as:
"How are you doing?" she asked.
I use lower case for "she." Is this correct? Or should it be capitalized, since "How are you doing" ends with punctuation that is not a comma?