Always strive to maintain the internal coherence of both mechanics and narration, as this allows both players and GM to gauge their actions within a communally accepted framework.
Be a fan of the party.
Take your time to set the scene, describe the surroundings, sights and sounds in sweeping detail. Strive for impressionistic, broad strokes.
Try to give every player time and space to contribute their ideas, and to develop their characters’ narrative in a meaningful way. Ideally, the GM should not take more time talking than any other participant.
Whatever you do, if at all possible avoid derailing any situation into YAMBS (yet another mindless battle scene).
To design good battles, keep the attention on a few significant features, as every change in position and composition needs to be communicated every round. Singular, challenging enemies are usually preferable over large swathes of trash mobs, as book-keeping them and informing the party about their actions is often simpler and quicker.
As GM you could (secrectly) choose not to track enemy health at all and instead conclude encounters at narrative peaks or when spirit and energy at the table decline.
Be sure to design all challenges based on the same scale and metrics. A task that was easy yesterday should still be easy tomorrow, variations in difficulty should depend on exciting external factors rather than whim.
If a given scene seems overly easy or difficult for the party, introduce an element of surprise! An unexpected ally might turn up at the last second to help out, new evidence might come to light at just the right moment, or a seemingly simple boss might suddenly turn into a raging hellbeast.
Always give your players the impression that you have a plan for What’s Next™ and that you are perfectly aware of the minute details of everything that’s going on. It doesn’t need to be true, it just needs to seem like it is. It’s fine to fudge a roll here and there, or to skip over some parts that seem out of place at a given moment.
If your story touches upon critical elements of either the game’s setting or other players’ stories, be sure to communicate conflicting details, in order not to violate the aura of internal consistency.
Use external materials like pictures, music, poems or puzzle boxes to enhance the experience.
None of this is set in stone! If you have a great idea that needs to be expressed, find a way to make it work with what you have and what you’re given. The defining feature of any RPG is the freedom of expression it allows all participants.