I was recently asked the question, “What’s more important a shoehorn or shoe trees?” That’s like asking what’s most important for your car motor oil, or gasoline.
In my humble opinion if this where a stranded on a desert island scenario I would say you are best to have a pair of shoe trees.
I do think that you should use a shoehorn when possible but if you are careful not to crush the heel counter of the shoes when you put them on they’ll be fine. On the other hand nothing will help preserve the life or fit of your shoes more than a pair of wooden shoe trees.
Lasted and sized trees made by the manufacturer are best if you can get them but if not the trees should fit reasonably well without being too large which will stretch the shoes or too small which will defeat the purpose of using them.
If trees aren’t an option, say for example due to travel you can substitute the day’s newspaper. Wad it up and stuff your shoes snugly. This is also recommended for shoes that are soaking wet. Stuff them full and if they have leather soles lay them on their sides to dry. Once they’re dry you can put the trees in.
One more recommendation is to be sure you rotate your shoes. Try not to wear them two days in a row. They need time to breath and they’ll feel much better the next time you slip them on.