It took me a while to learn proper form, 2 trainers later I felt like I knew a secret. I'd look around and see all the bad form and I felt like I was working out smarter and not harder. lol
I sure miss the trainer though, he'd catch my bad form and correct it. Unless I look in a mirror, I can't tell sometimes with free weights.
I can still hear my trainer telling me to drop my shoulders. lol
For me, and in my experience, form is like riding a bike. It's tough to forget. There are however those few exercises that if I don't do them, I get rusty. A huuuuggggeeee majority of people training in gyms today have piss poor form. I'd be willing to bet a huge majority of the members of this very forum has piss poor form. It's ashamed. There aren't a lot of resources (online and in gyms) that teach it.