WHAT CLINTON NEEDS TO DO FOR HERSELF:
Granted that the media has basically allowed, even aided and abetted, the Trump campaign to define Hillary Clinton to the public, she hasn't done herself any favors. Her campaign speeches are laundry lists of mostly small-ball detailed proposals with no common theme. Clinton needs to define that common theme, and it needs to be something beyond, "Vote for me to continue Obama's legacy" and "Vote for me because I'm not Trump." Without that theme, you don't really get the enthusaism needed for big turnout on election day... and it's been reported all over the place that Democratic voter enthusiasm is way, way down.
Trump has pulled close to Clinton both by dragging her down to his level and by offering his supporters a simple, easy-to-understand (and of course deceiving) message: "The Democrats weakened our country. We need a strong leader now to fix it, and that's going to be me." Clinton has nothing like that, and the debates are really her last chance to make that happen.
WHAT CLINTON NEEDS TO DO TO TRUMP:
Trump's biggest enemy is Trump himself. Clinton needs to lead him into making gaffes and factual errors, and that means pushing him off-balance and off-script. By all reports the best way to do this is to attack Trump's self-image. This can NOT be done head-on; Trump can shrug that off. It needs to be done in passing, a bit at a time, and carefully targeted- references to how he inherited wealth and nearly squandered it all on failed business deals, how most of his projects failed miserably, how his tax returns would show his lack of wealth, etc. This will get him angry and prone to counterattacks- and those counterattacks, though his base will love them, will destroy him with everyone else.
WHAT CLINTON NEEDS TO AVOID:
(1) Don't make the debate a referendum on Trump. If all Clinton does is give the case for why Trump is disqualified, then TRUMP WINS IN NOVEMBER. It's never sufficient for any candidate to simply say, "I'm not the other guy." Clinton needs to slip a few bits in about Trump, but her focus needs to be on why voters should choose her over any other option, even if Trump weren't in the race at all.
(2) Don't go full wonk. Clinton loves to try to teach people in her political messaging- even more so than Barack Obama. And it's boring. If Clinton details her specific proposals, the listeners will tune it out and absorb none of it. Clinton needs to stick to broad principles and intentions and address specifics only when unavoidable. KISS principle needs to be the guide.
(3) Don't try to go toe to toe with Trump in a shouting match. Responding is fine, if kept very brief and witty, but when Trump gets angry and attacks Clinton needs to stay calm and quiet. It's unfair but true that there is a lot of sexism involved in the electorate, and if they see an angry man and an angry woman going hammer-and-tongs at one another, a majority of Americans will by default side with the man. An angry man and a quiet but firm woman, on the other hand, goes the other direction.
Now for the part I hope nobody reads to The Donald:
WHAT TRUMP NEEDS TO DO FOR HIMSELF:
The Sarah Palin strategy. Palin managed a draw with Joe Biden in their 2008 debate by memorizing about a dozen talking points and steering every question to one of those. As a result, only people who were listening very carefully (I was driving and heard the whole debate on the radio) could tell how rehearsed and repetitive her answers were as opposed to Biden's. Trump needs to do the same thing: have a script of talking points and stick to it absolutely, positively, no matter what. If Trump improvises, he will be (justly) eaten alive for his misstatements of fact and his inevitable bigoted gaffes.
WHAT TRUMP NEEDS TO DO TO CLINTON:
The aforementioned script needs to have several very specific attack points on Hillary's honesty and integrity. Merely saying "Lyin' Hillary" might work on the stump to the base, but it won't fly in a debate. Trump needs to bring up specific cases with evidence for his accusations. And he needs to do this again, again, and again, dragging her down into the muck with her while appearing to be a prosecutor instead of a reckless mudslinger.
WHAT TRUMP NEEDS TO AVOID:
(1) Don't ad-lib. By all reports Trump has refused to do any prep work for the debates, insisting that he can do it ex tempore. Bad move. If Trump improvises, he gaffes.
(2) Don't miss the chance to denounce racism and bigotry. If Trump could go two weeks straight without himself or one of his surrogates making a white-supremacist remark or re-Tweet, he'd probably have this race sewn up by now. As it is, a substantial portion of normally Republican voters is currently supporting Clinton for the very reason that Trump is blatantly white supremacist. If he loses, this above all else will be what cost him the election (which is already looking like an exercise in whether a president can be elected anymore without a single non-white vote). If the opportunity comes to demonstrate that he hasn't got a white robe and hood in his closet, Trump must sieze it decisively and forcefully. If he hedges, or worse (as he has done elsewhere) he defends whatever white supremacism is at question, he fails.
(3) Don't lose your temper. Trump's charisma is based on the smiling, confident, loud Trump. The angry cry-baby Trump we see now and again will lose the election. Whatever provocation Clinton manages, Trump has to shrug it off as a lie from a desperate woman, beneath his notice. He must NOT try to shout her into submission. If Trump tries to bully Clinton as he did the other GOP candidates once upon a time, Clinton will walk away looking like a saint, leaving Trump exposed as the Satan he actually is.
WHAT GARY JOHNSON NEEDS TO DO:
Hope both candidates make serious blunders and discredit themselves, then hit every morning pundit show the next day hard and fast. Fifteen percent, Binky; it's your only hope, and you've admitted as much.
WHAT JILL STEIN NEEDS TO DO:
Quit running for public office and let someone with more credibility than a Scientology outreach flyer be the face of America's far left.