General:
Marth is all about spacing; when attacking, retreating, baiting, or defending. When your spacing is good, you can out zone your opponent, apply shield pressure, get tippers needed for kills, and occupy vital stage space. Movement (made of dash dances, wavedashes, and pivots) is crucial to spacing. If your movement is bad or sloppy, no matter how good you are otherwise, you will be punished. When playing high level players, don't forget that they are watching and learning all of your habits and will punish their overuse. Don't fall for dumb things; don't approach if they're baiting you in. Mindgames are real.
Specific:
Marth's grab is insane. Get grabs, make sure you capitalize on them either with a regrab, follow up (up air or up tilt), or techchase read
You don't have to be crazy awesome to edgeguard, sometimes all you need to do is grab the ledge. Other good options are dtilt, fsmash, and if appropriate ledge-hop dair
Tippered downsmash is a very clutch KO; comes out faster than fsmash and kills off the top. Make sure you tipper it though because it has a lot of end lag and is very punishable
An easy solution to aggro space animals is just to counter if you know they're going to approach with an attack (more risky on fox because getting grabbed by him is brutal).
A falcon offstage should be a dead falcon
one stock is not a lead
Don't get hit
super specific:
CrimsonBlur has some really good tips in this thread, read up
http://smashboards.com/threads/technical-things.283211/#post-10954335
I'm sure other smashers have more to say