I keep my pressed citrus oils in the refrigerator - they have a short shelf life.
Your trick of decanting out a small portion at a time to avoid opening the "mother" bottle is a good one.
Otherwise, it's good to keep headspace in the bottles to a minimum; I got some Oxblox from lotioncrafter (also available at wineshops) to use in larger bottles.
I hear you can get small glass beads to add to your bottles to bring the level up near the cap and reduce headspace - that might be the best since spraying Oxblox into tiny bottles would be quite a feat! It tends to result in splashing.