The big thing to check first is what's causing the cancellations. A lot of things you'll see get canceled (if you have a lot of notifications set anyhow) because of dwarves getting hungry or thirsty or sleepy and going off to take care of those needs.Things like "item no longer available" (I don't think that's the right message, but it's something along those lines) proooobably aren't a huge deal, since they can happen from various causes, and I don't usually seem to notice them causing a big problem.
Generally though if you get things like they can't find a route, or area inaccessible or something, then you might need to go look and make sure you didn't break a necessary route. You also need to watch out for job cancellations that won't automatically be re-queued - usually orders you queue up in the workshops are a big one (unless you have them set through the manager screen). If say you don't have any food for a cook to make prepared meals with, it'll cancel out any "create food" orders in the kitchen and you'll have to requeue.
Pretty much any job that uses items in a barrel, bag, or other container are a huge pain. Seeds, food stored in barrels, stuff you've got stored in bins... they're all annoying, because one dwarf will go grab the container to gather up new items to put in it, and that makes the items unavailable to any other dwarf. -_- Things like seeds being planted will requeue themselves, but like I said, other things like cooking you'll need to reset manually.