My son is 2 1/2 and monsters have recently appeared in our lives, too. A few things combined seem to be working for him...
1. Mom-Mom's the boss of the house and my rule is, nothing can come into our house and scare James. If James thinks he sees a monster and it scares him, tell the monster to leave, this is our house, not the monster's house. (We later discussed nice monsters like Elmo, Cookie Monster, etc, and he decided that if he sees a monster and it's nice, it can stay.)
2. James thought the monster lived downstairs, and would come upstairs and eat him in the dark. When he thought he heard the monsters coming, I'd hold him, or take him by the hand, and bring him downstairs (we have an open floor plan with a night light on either end, so there's some visibility downstairs) and ask him to show me where the monster was. I'd act all excited, like I couldn't wait to see the monster. He'd look around, and then look at me with complete confusion on his face and tell me he couldn't find the monster. If he had seen a monster, I would have turned on the light so he could see what it was a shadow or formation of.
3. During the day, we talked about the monsters. I'd ask where they are, when we were downstairs and it was light out, and he thought about it, and thought they were probably outside until dark, so we made sure we checked to see if the doors and windows were closed, so they "can't come in unless we let them in".
4. James has a picture of Jesus on his wall. He understands that Jesus is with him always and he can always ask Jesus about help if he's scared. (I understand this might not work with all families.)
5. We use "Monster Spray" before bedtime. It's just air freshener or watered down perfume, and he can spray it himself if he gets scared in the middle of the night.
6. We also (by coincidence) recently got a cat. My future stepdaughter told my son that cats and dogs scare monsters away, and I never would have planned it this way, but my son takes great comfort in the fact that our cat is "patrolling" the house while he sleeps and he's not so scared of monsters anymore, so this might have been what worked best for my son.