hi there -
Based on my personal experience JETBLUE always comes in less than everyone else (and has the best service too!

). We fly them from JFK - MCO and the most we've ever paid was $158 pp. They are the only airline we fly lately.
However, if we are going someplace that jetBlue doesn't, or if I'm checking prices for other people...
I always check the major airlines on their websites and then call them to compare. Usually the online prices are cheaper EXCEPT if I'm looking for a "senior" rate for my MIL and then calling seems to work best.
I haven't had much luck using any of the services like Expedia, Cheap Tickets, etc...I always seem to be able to find better bargains myself ( but it's always worth checking out).
Try
www.sidestep.com It's a free service and they find lots of cheapp flights for you (they search a lot of the lower/smaller airlines which is nice).
AS for how far in advance etc...to get tickets - ASAP is always my motto. AS for jetBlue -it is my understanding that they start their pricing at the lowest and when those seats sell they increase their price to the next level and continue up from there. However, if they offer a special promotion and you've already booked they will credit your jetBlue account the difference and you can use that $ to purchase tickets again within a year.
Hope this helps a bit...happy fare hunting!