#1 is to read Ca$hvertising, it's like the 101 handbook to writing copy (IMO) - it's a good look into human nature and learning why good copy actually works. (Don't stop reading here, but I would recommend starting here.)
#2 is to read a ton of copy, both good and bad. You'll learn from the good and learn what to avoid with the bad. Turn off your ad blocker RIGHT NOW and start clicking ads that you like. You will see everything from "DAMN that's good" all the way to "DAMN this is absolute garbage".
#3 is to practice, practice, practice - write whenever you can, and as davers said, start practicing your copy in real life sales/persuasion situations, too, even if the situation is completely trivial, like you're trying to convince someone to give you a ride in exchange for a coffee. Once you start doing it all of the time, it becomes second nature...
#4 is to read writing of all shapes and sizes so you can write effective copy for a wide range of products and services. For example, start reading complicated financial articles, "feel good" articles on sites like Elite Daily... everything! As you read, that type of "style" will become second nature, and eventually, you'll have almost every style under your belt. (In general, it's easiest to write copy in your natural tone of voice, so your non-natural tones might not be as good at the start - don't give up!)
That's what I could think of off the top of my head, hope it helps!
Jared