There are countless sub genres within fantasy, and each has it's own list of tips and tricks to look at. However, at the moment, we're just going to focus on the genre as a whole.
By Nidoart - Own work http://nidoart.blogspot.fr/, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=23756701
Here are my top tips to getting your fantasy work into shining condition:
Be bold. Don't shy away from making your world bold and real and vibrant. Your world is a fantasy, make it one! However, remember your world building rules. Keep your world in keeping with the rules and don't break them just for effect. You need to maintain the suspension of disbelief. This even applies for magic in the real world (say, in urban fantasy for example).
Know the conventions of your genre, but don't be afraid to mix them up or genre blend (just do it well!). Epic fantasy is normally quest based, but what if you mixed that up with something else? Keep your reader on their toes.
Don't get sidetracked by your world building. Writers either give a) way too much, or b) afraid of giving too much, they give way too little. Read your favorite fantasy books - how much do they give? Read recent fantasy books. Yes, we have our golden oldies, but see what the market is looking for right now.
Want some ideas of great modern fantasy books?
The Night Circus by Erin Morgenstern.
Miss Peregrine's Home for Peculiar Children by Ransom Riggs
Bone Gap by Laura Ruby
Six of Crows by Leigh Bardugo
The Accident Season by Moira Fowley-Doyle
As you can see, this is a tiny selection and there are SO many good books out there in the fantasy genre. Go read them!
Think about what modern teens care about? These things should be present in your modern fantasy. They might be in a different world, but the MC should still feel the way teens in the real world might feel. They need to be layered and complex, and modern!
Don't get complicated. It can be so easy for writers to create such a vast world in their head that it becomes super complicated (I know, I can be guilty of that), but the best advice I can give you is make it simple. Don't let your world (or plot) be overly complicated, especially if this is your debut. You want people to grasp your world easily and then they can focus on your amazing characters.
Alright, so that is a little bit to get you started. However, go web surfing. There are TONS of hints out there. Read your craft books. Experiment and have fun!
Happy writing my fellow fantasy people!!!