Trump: You're the outsider, act like it. Bait Clinton by calling a warmonger, attack her for supporting the "job-killing NAFTA and TPP deals", challenge her "extreme immigration [and refugee] policy". Undermine her responses to your criticisms by reminding the audience that "she's a liar, folks. Lied about Benghazi, lied about the emails, lied about rigging the primaries against Bernie Sanders. She's crooked, folks".
I agree with this. This is what makes the debates dangerous for Hillary. I don't believe that Trump should try to act presidential and serious. First off, he's not very good at it, second, he would get annihilated by Hillary in a serious policy oriented debate. But the relentless attaking and namecalling can definitely get Hillary off her game. She is at her worst when trying to defend herself and at her best when talking policy.
Hillary should do what she does best. Be a cold competent policy wonk. Don't get baited by Trump. Rebutt his attacks shortly and precisely and explain to the viewers why Trump is wrong about rising crime (it's falling) and the bad recovery (it is actually extremely impressive compared to almost all other western countries). And please don't try to be folksy. And don't laugh. Hillary shouldn't be looking for the love of the people but for the respect of the people.