I like rosemary and eucalyptus globulus. Add tea tree if my sinus problem is becoming a sinus infection. I make a steam tent by putting a few drops in near boiling water and leaning over it with a towel draped over my head. A diffuser works well too.
One of the LMP's I trade with puts a drop or two on a cotton ball under her face cradle to help keep supine client's sinuses clear. I do notice a difference.
If you or your clients use a neti pot to rinse their sinuses, I have heard a drop of tea tree added to the saline keeps infections at bay. I haven't tried this personally, though. But I'm sure I will before winter is over.

Chamomile is good for alleviating allergic reactions and very soothing. Mae, you may want to try including that along with the tea tree in the neti pot.

A mix of peppermint and lavender is wonderful for sinus pressure headaches. I like to apply the oils to the posterior cervical region up to the sub-occipitals and also on the forehead and base of the nose on each side. Take care not to get close to the eyes, the peppermint can be very irritating to sensitive tissues.

I agree with Mai, rosemary and eucalyptus are great for opening the sinuses.

Peace

There are many different essential oils that are useful for sinus oongestion/allergies, including eucalyptus radiata, sweet myrtle, bay laurel, basil ct. methyl chavicol, rosemary (any chemotype, although cineole is preferred), hyssop decumbens or peppermint. Try 2 or 3 of these in combination. I see that many people are recommending lavender here, but it's not going to help sinus problems or pollen allergies. Lavender has a mildly immunostimulant effect on the body, but is not indicated for upper or lower respiratory issues, unless you are using Lavandula stoechas, which is not recommended for beginners due to neurotoxicity.

-Kath Koeppen, registered clinical aromatherapist
www.aromaceuticals.com