There have been a lot of blog articles written about this - do some searches. Where Is Baer ?
has a pretty good write-up. I've never had an issue w/ just putting a paddle a ski bag and checking it, but sometimes extra charges can be an issue.
If you fly Southwest you're good to go (I think maybe Frontier too?) - but always read, print out, understand the official baggage rules. If you're flying many other airlines, there's no guarantee how it's going to go and arriving early is smart.
Know, the rules, dress up your boat to make it look like whatever you need to claim it is (such as, Waveski), and call ahead and get it printed on your reservation that you'll be bringing said equipment so when you check in and they give you shit you can say you got approval verbally and it's on your reservation. Be super friendly and tell the attendant that his/her mother must really love her.
I can't remember where I read this, but it was my favorite quote ever on the subject. Scott Lindgren was asked how he manages to fly with his boats, given the many expeditions he's done, and his answer was (paraphrased), "I walk up to the counter with my kayak and don't take no for an answer."