Let's start as a Realtor. I would know as I've been helping people in and out of homes for over a dozen years now.
1) Know what you're doing. Take continuing ed courses. Read the news, Watch what's happening in every field related to being a Realtor.
2) Treat people the way you want to be treated. Like, hopefully, the way your momma raised you.
3) Go above and beyond, it doesn't come natural to everyone.
4) Stay tuned into the market you service. Watch new listings, sold properties, price reductions. Every day, several times a day.
5) Keep in touch with everyone you know, including other Realtors.
6) Create a good team of affiliates so your clients are treated by them the same way they are treated by you.
As A Seller?
You're going to laugh since it's so very easy. Look at the list above, hire a Realtor like that described, DO WHAT THEY TELL YOU TO DO! Yes, it's truly that simple.
As a Buyer?
Unfortunately, not quite as simple. Close, but follow these suggestions:
1) Make your list of wants/needs.
2) Expect some heartache.
3) Do what you Realtor tells you to do. Of course that's after hiring one that is like the one described in the first section.
Yep, pretty much that's it. See, Easy Peasy.