We all love to read about journeys because life is itself a journey. The quest story has been popular for many centuries. From "Sir Gawain and the Green Knight" to the Harry Potter series, writers have taken us on journeys full of adventure. But they also teach us something about ourselves and the world we live in.
Take the time to develop your characters. Traditionally the quest story has involved a hero as the protagonist with the heroine either left at home or the reward at the end of the journey. He's helped on his adventure by a companion who acts as a sounding board for his dilemmas.
Make sure the object or person sought is compelling enough to warrant a journey. Something sets the hero off on his quest and he must stick it out until he reaches the end. Odysseus, for example, has triumphed in the war against Troy and must return to his home, wife and son no matter what adversity he has to face on the way.
Plan the complicating incidents that will prevent your hero from reaching the end of his journey. These should move the story along at a rapid pace. Quests by nature have a lot of action. It's not necessary to introduce supernatural elements if your story doesn't warrant it. But there must be people and forces that work to keep your hero from finding what he's seeking.
Remember that a quest story is essentially a search for wisdom. Your protagonist must be different when he gets to the end of his journey. Often wisdom is passed on to him from older characters. He must also fail and make amends because the hero, like all protagonists, is really each one of us.
Spend time working on your ending. It must give closure or readers will feel like they've wasted their time. Often what the hero finds at the end of a quest isn't what he anticipated he'd find. Make sure there's success in the end even if he doesn't get what he wants.
In a story where the romance is important, make the heroine seek the object or person that the hero seeks but for different reasons. Create conflict between their motivations.
Don't let compelling minor characters or feats of adventure sidetrack your hero. When revising make sure the focus remains on what he's trying to find and the wisdom he learns on the way.