This is pretty simple:
Two (or however many... just not eighteen!) candidates.
One moderator who, as the title implies, moderates the discussion. S/he has the power, up to turning off the candidate's microphone, to see that rules are obeyed.
Moderator controls who talks at all time. Moderator also controls how long candidates may talk. All controls are done in a manner that doesn't interrupt the discussion: silent buzzers, offstage signals, what have you.
After initial layout of rules, moderator is not seen again until end of debate. Not even the back of the head. Sorry, TV guys, this ain't about you. Moderator will only be heard if s/he warns a candidate that they are not obeying the rules.
One issue per debate, rather than a scattershot of issues.
Each candidate given three minutes initially to state their policies on the issue.
After this, it is up to the candidates to continue the discussion. They're all lawyers, they all know how to debate.
They are allowed to question each other's policies and the facts and opinions they use to support them.
All questions must be questions, rather than comments with a question mark tacked on the end.
Once queried, a candidate has as much time as necessary to respond.
Candidates wishing to interject signal the moderator, who decides when and if an interjection is allowed.
Moderator may take the floor away from any candidate at any time, for the following reasons:
1. Rehashing a stump speech rather than continuing the current thread of the debate.
2. Making a personal attack; every candidate knows how to questions another's opinion without attacking the other's person.
3. Resorting to emotional appeals "I was talking to a blind widow in Annapolis today who told me..." or pandering to voter groups rather than sticking to the issues and why they believe their policy will be effective.
4. And, of course, just talking too damn long.
It would be interesting to see if they could actually pull this off, since most of them probably haven't had to do this kind of debating since college.
If it got too hard, you could give an exemption every fifth or sixth debate, and they could go back to the stump speeches and personal attacks that everybody likes so much.