For headers to be the most effective you have to address the whole system with big enough exhaust pipes, an x-pipe and good flowing mufflers. You should also stay with a good quality set like Hooker or Hedman. If you start out with some econo headers they won't fit quite right, the flanges are made cheaply and thats where a lot of the leak problems are.
I have had the same set of headers on my car for 10 years now. I check the snug on my header bolts from time to time but they don't leak. About once every year or so based on how they look I take them off, clean them up and re-spray them with black stove paint and put new gaskets on. Thats about it.
The only big downside with headers is that they allow more heat to escape into the engine area. It can actually make your floor panels hotter. I've been told that the new ceramic coatings help this a lot but those are a bit more expensive.
You can see my 65 fastback at:
http://hottarod.stangnet.com/
http://members.aol.com/macstang/Gifs/macstang3.jpg