Toasty:
I've played a few pretty good marths, and I had the same trouble as you. And, admittedly, I am still not incredibly adept at returning to the stage against competant Marth's, but- I will offer what advice I can. Of course, anyone feel free to correct me if my reasoning is flawed!
What I've noticed against an intelligent Marth, is that they don't really do anything to totally KO you as you are coming back- aside from the occasional spike. But, I've cancelled their spike below stage level, at around 130%, and still lived to fight on. So, if they fail a spike or two, they might be a bit hesitant to do that again, which is good- because it gives you one less thing to worry about- or in the very least moves the straight up spike down the list of possibilities.
As I said, they don't really try to go for the KO... just annoy the piss out of you until you use all of your fuel up.
If you are recovering from above, don't randomly throw out a laser or gyro. Be meticulous about it. If they come off the stage with a jump to hit you, try to catch them in the jump animation. Their options: Airdodge, and therefore give you a second or so to make up some ground. Get hit, which gives you some time. Aerial through the gyro, wich again can give you more time. Dun worry about a spike here, as anyone can probably cancel it and get to the edge. Infact, its almost to your benefit if they do. It will force you down extremely quickly *closer to the safety of the ledge* while they are stuck in their cool-down.
Recovering horizontally and from below is the worst, IMO. Horizontally is bad- because you will be susceptible to an F-air chain, as well as ledgehopped B-airs. Same goes from below for the most part. My advice, time a gyro or laser to surprise them if they hang on the ledge for too long and lose their invincibility frames. Otherwise, stall and cancel ur recovery with f-airs and N-airs, thereby throwing off their timing at edgeguarding you, and giving you a narrow opportunity to get back.
As far as recovering from below: watch for the spike, the ledgedrop to Up+B stage spike to ledgegrab, ledgedropped B-airs. Your options are limited here, about the best you can do is fire a diagnal up laser. And, if your directly below them- try a rising U-air.
Hope that helps some.