Readers are entitled to believe that anything appearing within direct quotes are the actual words used by the speaker.

• Remove ums and aahs and correct for grammar 
• If you are unsure of the exact wording use reported speech 
• Open quotes with a colon, and close them after the punctuation mark
• Use says, not said (though said is permissible for famous historical examples)

"I really enjoyed the part about the accelerators," says Joe Bloggs. "It got me interested in physics again."

Martin Luther King said: "I have a dream."

Do not use words like "exclaims" or "complains" – they interpret the mood of the speaker. We are reporting what people say – we can never know exactly what the speaker is thinking when they say something. Likewise, do not use words like "adds" and "replies" – speakers can only add or reply to a comment if they are present at the discussion.

So avoid:

"The Higgs boson is an enigma," says Ben Dacre at Fermilab near Chicago. "It's hard to detect so we may never find it," adds Jaques Dupont at the Université de Paris Sorbonne.

Avoid adding quotes which do not add information or, at the very least, "colour" to the story. Though contributors may be anxious to see their names in print, such quotes are tiresome for the reader. Focus on the story, not the interviewee's ego.

AVOID:

"This is an important result because it raises new questions," says CMS spokesperson John Doe. ATLAS spokesperson Joe Bloggs says: "We have many new questions to think about based on this result." Jenny Smith says: "This result will keep us questioning our theories for a long time."

etc.

Finally, be careful about punctuation in direct speech. See this useful guide from the OED.