Tips/Advice for a Playthrough of Pokemon Omega Ruby/Alpha Sapphire?

Question

A thread for posting (spoiler-free) advice for a playthrough of the given game which will help anyone playing it. The aim of this thread is to make playthroughs easier for someone who is playing it for the first time.

A couple of rules:

1. Don't give advice for Pokemon as a whole. Seperate thread for that can be found here.
2. Try to keep advice beginner friendly. Do not go into the more complex mechanics.
3. Don't post multiple answers. Edit your existing answers to add more.
4. No repetition

Comments

I would recommend using pelliper. Its a powerfull Pokemon and it can demolish some gyms, really helped me during emerald.