It's very common to add a bit of jojoba oil to all of your blends (once the essential oils are added to the carriers) to lengthen the lifespans of the other oils (both essential and carrier). And... jojoba oil just freakin' rocks all around.

Keeping all of the oils (especially the essentials) away from light and air is your best bet. So, in addition to keeping them in the dark amber bottles, I also put mine in a small wooden box/chest to completely block out light while they aren't in use, and I always avoid leaving the bottles in direct sunlight when I am working with them. Obviously, it's difficult to prevent the oil's exposure to air, but making absolutely sure that each bottle is capped tightly after use will help. When you're actively using them, even if you know that you'll be going back to that particular oil for a few more drops in a blend, cap the bottle tightly in between.

That's about all you can do! Although some do go rancid, and their smells noticeably change, I haven't yet smelled an essential oil that has become an "odor" after it expired. I'm not sure how powerful their medicinal effects are after going rancid, but I don't see why they cannot still be used magically.