Become one with your controller. Learn to use all the buttons on the controller equally, kind of like spreading butter on toast. It has to be just right.
Then the next step is to become one with your character, FEEL your character get hit, don't just see it happen. At first use rumble to get a hang of it. But then when you become more advanced you will become your character. Like evolving from a metapod to a butterfree. Especially since i was just talking about toast.
Then you must learn intimidation, combos will only work effectively if you can intimidate your opponent and immobilize them by striking fear into them.
After all of these have been accomplished, only then can you improve.
lmao that's some deep ****
Here's my advice. This should get you better faster than anything else:
First, learn to perform all aerial attacks with the C-stick. Do not use A at all! Ever! (for now) Second, learn to grab more. Since you're going to learn to grab more, you might as well start out doing JC (jump-cancelled) grabs since they are really very easy. Third, learn to L-cancel all of your aerial attacks so that you rarely miss one, no matter what the timing. Fourth, learn to face the stage then jump backwards to grab the ledge and edge hog your opponent.
That's it, except I forgot to tell you to learn to tech when you get hit and land on the stage so you stand up immediately. Just do it by press L (all the way in) before you land on the ground.
If you didn't understand the terms I used or how to do anything I said, search on the melee discussion forums or marth boards to learn about them. Read all the stickied threads.
Good luck