FTilt outranges everything Marth has. Smart use of it helps miles in the MU.
DON'T FORCE THE KILL. This is the worst possible thing you can do. Marth is very strong defensively.
DON'T RETREAT UNLESS YOU ABSOLUTELY HAVE TO. You want every inch of stage you can get to pressure him.
IF HE'S CHARGING SHIELDBREAKER DON'T BE DUMB AND SHIELD OR SPOTDODGE. Just something I see too many newer players do a lot... I did it a lot myself for a while.
Exception to rule number 2: Your shield is almost broken. Having your shield broken can lead to death. Marth does decent shield damage. If you're nearly shield broken, try and get some room to have it restore a little. Common sense, but I might as well say it.
Marth is a +1, maybe a +2, but it's a hard positive.