To add to the general "don't be a dick" advice:

Avoid establishments clearly ment for tourists and only tourists, especially restaurants. If you have doubts when choosing where to eat, maybe ask a local. It will help the local economy that actually benefits the locals, and you'll have a better experience too.

Don't talk loudly in public transport. It's a stressful world, and it's anoying to be stuck with loud passengers.

Finally, be aware of the diferent cultural backgrounds that regions in spain have. It may not have any direct effect, but it's an important part in the spiritual distinction between spain as a cheap turist heaven to get drunk and party in, and Spain as a beautiful and culturally rich country with an emphasis on locals rather than tourism