How is the rest of your game? If its only the long irons you're having problems with, have you thought about switching to hybrids to replace them?
Other than that, unless someone watches your swing it could be difficult to determine why you're hitting your long irons fat. There are a lot of instructional books that could help, as well as on-line instruction sites that could provide basic help.
well, i just started hitting my 4 and 3 iron and they are suprisingly having a nice ball flight.
try putting the ball just forward of the middle of your stance and make sure you take a normal swing as your 8 iron (speed, grip pressure, etc.). make sure you dont try to help it up. in the words of bobby jones, "don't fight the ball, let the club do some of the work. if you need any more help go to my thread "do you need help" and i'll answer it
Practice hitting range balls and feel what a good shot feels like. Experimenting, trial and error at the range worked great for me. I think the best way for anyone to learn a good golf swing is to learn to hit their long irons. Hitting a 2 or 3 iron is a really superb diagnostic for how good your swing is. For me, I watched a lot of television and then headed to the driving range while the ideas were fresh in my head.
Hybrids are a good option, obviously. But if you can learn to hit your 2 or 3 iron, there's no better feeling than knowing you can blast one on a tight par 4 and find the fairway or at least keep it in play. There's nothing quite like hitting an iron in my book.
go to hybrids... but you can still learn how to hit your long irons.. I'd say start punching them.. honestly.. just put the ball in the back of your stance and hit down on them.. they fly a little lower but I hit punch irons a little longer... then once you build some confidence.. put the ball in the front of your stance and just swing with confidence.
If your hitting it fat (Which i thought was hitting the ground first) you will still do it with hybrids as its a swing fault.
It took me a while to be able to use my 3,4,5 Irons. Try a session at the range starting with the 9i and work down the clubs after hitting 5 balls with each.
3,4 Irons i have just slighty to the left of middle, 5,6 right in the middle.