Find a message. And stick to it.
Trump's approach during the Republican primary was to try a series of attacks (and nicknames) against his rivals until one stuck. And, one always wound up sticking - "Little" Marco, "low energy" Jeb and so on. A general election is a different animal; you can't just throw 100 messages out every day and hope one lands. You need to decide the two or three things you really want to emphasise on a given day, week or month and then talk about them every day, all day. For Trump, that should be: a) the need for real change in Washington and politics b) the need to elect him to ensure a conservative-minded Supreme Court and c) Clinton can't be trusted. That trio of issues/messages would resonate with a broad swath of the country. And, in most of his speeches and interviews, Trump mentions one, two or even all three of them. But he also mentions 200 or so other things, making it very hard for a would-be voter to cut through the clutter.
Stop picking dumb fights
There is no strategy in which Trump's almost-week-long back-and-forth with the Khan family was a good idea. You empathise with a family who have lost a son in combat, not try to demonise them. This fight was a political cul-de-sac from which Trump had no reasonable exit strategy. Trump's natural inclination is to attack when threatened. Always. He doesn't believe in taking the high road. Ever. But, any candidate (or adult) knows there are fights you want or need to have and those that you should avoid because they aren't ones you can win. Trump seems convinced the fight is enough - even if he loses, because, in his mind, he never loses. The Khan episode should be instructive to Trump. Rather than blaming the media for covering it unfairly, he should use it as a teachable moment. Pick fights you can win. Walk away from all the other ones.