There are some long-acting decongestants on the market.
If you want the official medical line, waste some time and call up a doctor. They will walk the fence and say that no significant research has been done on the effect of medicine on the human body at depth and it is thus not recommended. They will definitely say you should not take decongestants as there was a study linking them to convulsions.
Read that DAN link once again. See how there are no set conclusions and recommendations, just some vague mumbo-jumbo? That is because doctors in general cannot give blanket recommendations, and prefer instead to err on the side of caution.
You have to understand that with diving, just like anything in life really, you are the only one responsible for your well-being.
You have to find out for yourself what works for you.