What sort of trends in daily peak flow readings distinguish asthma/COPD from normal breathing?

That's a very good question. And something every asthmatic/COPD pt. should be aware of.
It's mostly to do with correlations so keeping a journal next to your daily readings will give valuable information not only for you but your doctor as well.
Let's say your normal peak flow, and by normal, I will not give you a number, is a certain #. You establish that on days when you aren't short of breath and before you take your inhaler or albuterol treatment. When you are short of breath(sob) measure peak flow(pf) before and after your use of a bronchodilator. This will give you an idea of your response to that particular bronchodilator. Make sure you allow time for the medicine to work before you take the reading after the treatment. That means at least 5 minutes. If you are still feeling sob after the first dose then do another and again wait 5 min.The medicine must have time to be absorbed and circulated. Bronchodilators are absorbed and do not have a direct effect on the airways. They are meant to relax the bronchial muscles so they have to get into the blood stream before they can get to those muscles. What does have a direct effect is the saline that it's mixed with in the treatment form as opposed to the inhaler which is undiluted albuterol.. Sorry for the sidetrack.
Ok, so make a note of what it took to increase your pf readings. Like one or two puffs. Or one or two or three treatments. Btw if your using your inhaler as a regular routine and not for acute sob always allow 5 minutes between puffs. Again, that allows for maximum effect and the second puff gets much deeper than it would if you were to do one right after the other.
If you are asthmatic you might have much higher readings normally when you are not sob than COPD pts. who have chronically low readings all the time and have less response to brochodilators than asthmatics. That's just the nature of the condition.
As I said keeping a log of these readings over time help you keep track of what's working and what's not. It may inspire your doctor to put you on steroids as a means to improve your daily readings and life in general. Yes there are side effects of steroids but they are manageable. That's a subject for another time.
I hope I've given you some understanding of how to benefit from your peak flow readings. If you need further info I'll be glad to help anytime.
God bless.