Here are my two cents:
1. Use f-tilt. It's the only move whose range can compete with Marth's sword (and which is not so slow it will get you killed). Use missiles like everyone else has been saying ... but in the end, Marth can cut through these at will. The f-tilt (in combo with the wavedash for spacing) is what you fall back on to fend off Marth's approaches. Just watch how liberally HugS throws out f-tilts vs. Taj:
http://www.youtube.com/watch?v=75SsAl0BysI
2. Stay out of the air. Marth controls the air, period. Only do aerials if you're sure your move will land.
3. If you can, avoid Final Destination. There are no platforms to help you escape Marth's aerial attack.
And, a more general piece of advice I like to give people: don't be afraid to grapple. Less experienced players shy away from it because they used it incorrectly (and were punished for it) in their early days. If you play against someone good and never grapple, you'll notice a lot of your moves getting blocked. That's because if they see a move coming, they KNOW they can simply block and not get hurt. When this starts to happen, start to grapple.