Guaifenesin is magical stuff, but it's not going to save you from an earache. The main effect of guaifenesin is to help you cough up junk that may be accumulating in your lungs. What will probably make the difference between a tollerable flight and a busted eardrum (at least, for me, and I've had plenty of problems in the past):

- a decongestant (e.g., sudafed)
- a nasal steroid (anti-inflamatory) spray (e.g., nasonex or flonase)
- saline spray (not actually a drug, just keeps things moist and happy)
- maybe, an antihistamine (if and only if you've got allergy issues)

In the U.S., at least, nasal steroids are prescription-only. They're positively fantastic and have no significant side-effects. You squirt in the morning, and it lasts all day, but you have to do it for several days running to have any meaningful effect. When you're sick, all the little passages in your head both fill with gunk and get inflamed and constrict themselves. Nasal steroids help undo the constriction and allows the gunk to exit by itself. These drugs have completely solved my previously severe allergy problems.

Decongestants, antihistamines, and expectorants (like guaifenesin) are often combined together with general pain meds like tylenol. If you look at the back of the box on all the usual "cold and flu" drugs, you'll usually see a combination of these drugs. In the U.S., they're all over the counter. In Europe, good luck. I tried to buy sudafed when I was stuck, sick, in Amsterdam many years ago and the pharmacist told me it was prescription-only. "So, I can buy all the pot I want, but not a decongestant?" Of course, for N different ingredients, that means there are 2^N different combinations, and you really can find all 2^N sitting on the shelves of most pharmacies.

Sudafed, regardless of how it's packaged, typically comes in three different doses. There's the two little pills that last you three hours, then there's the one big pill that lasts twelve, and then there's the huge pill that lasts a solid day. When flying, your best bet may be the three-hour variety because you can time the dosing more precisely. Assuming 10-15 minutes to reach full effect, you can take it right when you get on the plane and ~45 minutes prior to landing. Assuming that initial descent begins 30 minutes prior to landing, you want the drugs humming along nicely before descent (and cabin re-pressurization) begins.

The saline spray is a simple addition. It's not a drug. It's literally salt and water (and a preservative) in a squeeze bottle. Shake and snort as often as you like. Airplanes are incredibly dry, and by hydrating your head, you can both drain gunk and avoid irritation, all of which is strictly helpful to keeping your eardrums from going boom.