1. Understand that essential oils are not the same as perfumes
Most perfumes are made from synthetic ingredients, even when their packaging claims that they are "natural" fragrances. They may used words like, pure essences, fragrance oil or aroma scent oil. There are also water based essential oil. Pure essential are not water soluble. In order to for water to blend with water, an emulsifier will be needed to add into the essential oil. If you are planning to buy an oil for therapeutic purposes, be careful to check if the oil you getting is pure essential oil.
2. Don't shop exclusively by price
Discount oils may not be as pure as more expensive ones, and it's also true that most expensive oils are not necessarily the best. At handful of distilleries sell their essential oils to bottlers all over the world. It's possible that you are buying for higher price from a fancier label or multi level marketing scheme. I would suggest you buying some common oil (tea tree or eucalyptus oil) to test on the brand you haven't tried, before getting the essential oil that are more costly.
3. Remember that essential oils are highly concentrated
Just a drop or two of most essential oils will be more than enough to produce the desire effect. Doesn't mean the more will have a better result.
4. Add essential oils to carrier oils
Because essential oils are so concentrated, they can cause skin irritation and other adverse effects if you apply them directly to your skin. A carrier oil can be use to dilute a highly concentrated, potentially irritating essential oil and carry it to the skin.
5. Keep essential oil in glass bottles only
Essential oil can eat through plastic containers. For 100% essential oil should be stored in glass bottles. Amber or cobalt bottles can prevent the oil getting the sunlight. If you making some mist or spray with some essential oil, you can use PET plastic spray.