Well, first of all, it's important that you know the risks you are taking by using decongestants in order to be able to dive. Should they wear off while you are underwater, you will at best have a painful and unpleasant ascent, and at worst, rupture an eardrum or do other significant damage to your inner ear. This is why the use of decongestants is not officially recommended.
However, they are commonly used. There are two basic types -- oral, systemic drugs (that are in the bloodstream and affect other organs as well as the sinuses and ears), and topical sprays. All of them act as vasoconstrictors (making blood vessels narrow down) as their mechanism of action.
Systemic vasoconstrictors have some side effects, like raising blood pressure and heart rate, that can be of concern for people who have blood pressure or cardiac problems. In addition, there is anecdotal evidence that systemic vasoconstrictors (eg. pseudoephedrine) can raise the risk of oxygen toxicity when using enriched mixes. Sudafed, one of the common brands, in its standard preparation, is fairly quick acting but short-lived, and I personally use the time-release formulation when I feel I need a little help to keep my ears clearing.
Topical vasoconstrictors do not have as much systemic effect, and are very powerful at relieving congestion. They may or may not relieve ear-clearing problems, though, depending on whether you are able to deliver the drug to the tissue around the orifice of the Eustachian tube. They tend to be marketed in a long-acting formulation, whether it's neo-synephrine or oxymetazoline (Afrin). The big risk with topical decongestants is rebound and the vicious circle that leads to rhinitis medicamentosa. This is where you use the drug, and when it wears off, the congestion gets worse because the vasoconstriction itself has caused the tissues to give off signals to increase the blood supply. You then reuse the drug before the recommended interval has passed, and each time, it wears off a little sooner, and you reuse it sooner. Sometimes it can require steroids to get someone off Afrin, if they've been abusing it regularly. I recommend to my (non-diving) patients that they use half the dose, half as often as it's stated on the label, and accept that, in between doses, they may have increased congestion.
Please do not take this as any endorsement of the use of decongestants for diving. It's simply information you can use as you see fit.
