I've gotten more than my share of head colds and have been in this situation on many an ocasion. A call to your Doctor will really help to set your mind at ease and only he/she can offer advice tailored specifically for you and your current situation.
That said, here's the sum total of Doctors orders I've gotten over the years on the subject.
Keep taking the decongestant as often as prescribed and in the maximum dosage allowable. If you haven't already, start now drinking LOTS AND LOTS AND LOTS of water. This is a big one. Goes hand in hand with the decongestant as all that water will thin out the fluids in your sinuses for easier drainage.
Bring a nasal spray decongestant in case you experience any dramatic discomfort. The advantage of the spray being that, in a pinch, it will take effect immediately. Ask your Doctor for more specific advice about this when you talk to him/her. If you have discomfort it will more likely be during descent than ascent.
Fianally,
www.earplanes.com can be bought at any pharmacy for about $5 a pack. These things are brilliant. It's a small earplug filter that slows the presure change so your head can catch up. They're comfortable, not too silly looking, and most importantly, they really work.
Again though, if you haven't already asked your Doctor specifically whether he/she thought you should fly at all in your current condition you should do that first. Provided they feel it's safe for you to fly, then it just bcomes a comfort issue and they can give you the best advice for what will work for you personally.