Keeping It Fresh

A Passion for Running the Game
Referee enthusiasm transfers to player enthusiasm. A referee who does not look forward to playing will doom a campaign quicker than most other things. So how, as referee, do we maintain our excitement for running game sessions, especially over the long haul?
My first goal is to run a setting I really like. I am particularly fond of my own homebrew setting, but there are also several published settings that I also enjoy. Each player in the game has one or more characters. The setting serves a similar purpose for the referee and is effectively their "character(s)".
The "monster manual" is full of creatures, some will appeal to you more than others. As referee you can, and probably should, run the monsters you like. Include favorite monsters in your random encounter tables and set encounters. Think about how the monsters will behave and what motivates them. Decide on a few favorite tactical methods for those creatures and give them "personality". Don't be afraid to "role-play" your monsters.
Design challenging situations for the player characters that will entertain you as referee. It's fun to watch how the players approach different challenges and as we all know, players will frequently think of things we referees never imagined. Go with it! That is part of the fun for all. Let the players surprise you and try to not paint yourself (an them) into corners by devising a challenge with only one solution.
Fill your sessions with interesting non-player characters. The NPCs are often what defines the world setting. Give the NPC a name whenever possible and keep notes. That way the campaign stays real and the burden to remember is lessened. A few words describing key features, mannerisms, beliefs or motivations can help you bring the NPC to life. Have fun with them, and again, follow the players lead. That takes some of the creative burden off you as referee and the empowerment players will feel will add to their enthusiasm and yours.
Encourage your players to bring their characters to life through role-play. The mechanics of characters can become boring, so ask them to focus on developing "character", i.e. personality. Have a discussion with your players about what motivates their characters, what challenges and rewards your players are interested in seeing in the game and work to make that a part of your campaign. Over the years of refereeing various systems I have learned to appreciate including character Passions (from Mythras and RuneQuest), Beliefs (Burning Wheel) and Vows (Ironsworn) into games like Pathfinder, White Box D&D and others. A good idea is a good idea and worth borrowing for your game.
Monster stats alone rarely make interesting encounters. Combat can become routine and boring if there is little role-play taking place as part of those encounters. The hobby has come to be called role-playing for a good reason. Role-play brings all aspects of the game to life and provides most of the "entertainment". By facilitating role-play at most every opportunity the referee and players are more likely to enjoy the campaign and want it to continue - continue to play to find out what happens.