- unless the tag is needed to add a particular nuance to the sentence, eg "whispered," avoid getting too creative with dialogue tags. "Said" should be used most of the time. It becomes almost invisible to readers, while other dialogue tags draw attention (and not necessarily in a good way!)
- you don't need to say "Bert said" every time during a conversation between characters. Readers should know who is speaking through the voice used by the two speakers. If it isn't clear when you remove extra dialogue tags, revision may be needed
- don't just shove backstory in dialogue as a sneaky way of getting it into the chapter. Readers will know, and agents won't approve!
- break up dialogue with action. People rarely stand still when they talk, they usually fidget or do something. Make sure you put action beats into your blocks of dialogue
- please, please, please don't have your character talk in slang or dialect through the whole book. That nuance can be introduced in a different way early in the story - readers will quickly grow frustrated (and stop reading) when they have to try to work out what the characters actually mean when they say "Avvagoadthat Luv"!