Doing groundwork I'd say is the best way to earn your horses trust. It requires patience and time but the end results you'll achieve are incredible.
To earn trust you need to ensure that your horse knows that you are it's 'herd' leader, just like it would trust the top horse in a herd to protect it. To do this, I'd start with round-penning and moving a horses feet to earn its respect.
Once you have your horses respect and full attention (it moves away from you immediately when asked to do so) do lots and lots of ground work and desensitisation. Desensitisation is great to make your horse a lot quieter and more trusting around you in environments and situations where they would usually be frightened.
If you watch Clinton Anderson's videos on YouTube on ground work, respect and desensitising a horse to scary situations, you'll gather more of an understanding of how a horses brain works and the fundamental things required to earn their trust and respect on the ground.