All you do is have them slotted on your ability bar and make sure it's the bar you're currently using when you gain experience points. Whenever you gain experience it only counts toward the bar that you're currently on, so if you're on your primary ability bar the skills you have slotted on your back bar won't get any xp.

Besides that, I have noticed some do seem to take more xp to rank up than others, I dunno why.