They have to have some sort of structure to them, even if its just a beggining, middle, end type of thing.
Seriously. A really good, although hard to find, example is "Sun Drenched Snow Cave" but Boris/Merzbow. It may be 64+ mins long

but it has structure. 3 definate parts to that song though. What makes in interesting to me is that all the parts are diff lengths, and the fact that they blended a more traditional rock type intro (albeit a long one) into a noise middle, and drone ending, IIRC. Been a while since I've listened to that particular recording
The other thing that interests me about that particular composition is how the 3 parts are different enough to stand on their own, but have enough simmilarities, that together they make one great listen. Not only should you move any of you're pieces through multiple (if even just 2), linked sections, but each section should have a rhythm all it's own on which it can stand solo, if need be, but still be linkable.
Having a good solid set of bass lines is the key to this IMO, and I hope some of this random rambling helps in your situation
