I don't want you to go without an answer though I was hoping to get some additional information.
Dogs don't get colds like humans, but they can develop upper respiratory illnesses such as kennel cough and canine influenza as well as bronchitis and inhaled allergies. You indicate that it has been going on a long time which likely rules out illnesses such as bronchitis and influenza and probably kennel cough as the illness would either resolve or get much worse. You can read about bronchitis here:
It’s possible that if there is also sneezing and nasal congestion, it can be due to an allergic reaction to an inhaled substance. If this is the case, Benadryl can be given to your dog, the dose is up to 2mg per pound every 8 hours.
You can read about these here.
Kennel cough is normally contracted when a dog has been boarded or kenneled or around a large number of dogs such as at a dog show, dog park or pet store. Here is a website with more information on kennel cough. http://www.peteducation.com/article.cfm?c=2+2096&aid=452
Canine influenza is now becoming more prevalent and like it sounds it is a canine flu. Here is an excellent site on it.
You will want to monitor your dogs condition looking for colored discharge from the nose or eyes, a productive cough (coughs stuff up), stops eating or lethargy. These are signs of a possible bacterial infection as well and my require antibiotics. If your dog appears to be having a difficult time breathing, you will need to see your Vet as some dogs dog get really sick with canine influenza and need support to recover.
To help your dog breathe easier you can run a NON-medicated humidifier in the room your dog is in, or sit in a steamed up bathroom with your dog to help keep the mucous moving. Robitussin DM at a 1/4 teaspoon per 5 pounds can be used to control the cough. It should only contain dextromethorphan and Guaifenisen. and should NOT contain pseudoephedrine. Dosages can be obtained at this website. However, since the cought might be related to an undiagnosed heart condition, don't give until that has been checked by your vet. The medication can mask a heart condition.
There are other reasons for coughs such as heart problems which should be investigated since she is an older dog. The following site goes over canine coughs.
I suspect some of her coughing through the years may have been due to other reasons like reverse sneezes or collapsing trachea. Here is a site on reverse sneezes and one on collapsing tracheas.
Given her age, I'd have Mady checked by your vet for a possible heart condition. I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.