If your dog is dealing with a complicated or long term medical condition, you might find it useful to keep track of his symptoms and progress in a visual chart.
I started putting some information down when we were dealing with Jasmine's chronically bad stools. I found that I might remember what happened a day or two back, but that's about how far back I could recall things accurately.
First I started keeping a journal, but I found it was quite overwhelming and it was really hard to see progress and compare one day/week to another. I figured that keeping a visual tool where I could see in a glance what went on in a past week or month would be helpful.
This tool has been helping us for a long time now. I can see how she felt from day to day, what she ate, what medication she was on etc. I can see how each treatment might or might not be working, whether there are other aspects affecting her condition (such as weather or air quality). I can see how much progress we made from the previous month or the previous year.
Our vets and myself find this to be a very helpful tool. Because Jasmine's issue is tricky, my chart got quite complex, as we are looking for a clear pattern. Of course you only need to keep track of the things that you and your vet find relevant for the issue your dog is dealing with.
Our vet has a similar blank chart available on his website to track quality of life for dogs who are dealing with severe arthritis and other conditions that have profound effect on quality of life. This again is a tool to see how well the treatment is working and whether medication or strategy needs to be adjusted.
If you and your dog are lucky, you will never need such tool. But if you and your dog are struggling with a chronic health issue you will find it useful.