What I have found through experience, especially listening to others' opinions in Writers' Workshops on stories that need tension, the most suggested was to shorten sentences.
Now, just saying shortened sentences might not give you much idea...so here is an example from my own novel.
To explain, it is a dance scene, a jig to be exact, and the MC is dancing in the middle of a circle of people. It's a "How long can you dance?" dance off. This village does this every autumn.
Even though this is a dancing scene you can (hopefully) feel the dancing. Another way to increase tension is to increase verbs.
He twirled. Skipped, and jumped in place. There was cheer, another man down! Belome was sweating. The sweat poured into the collar of his shirt. He left his spot to dance around the ring. Hands reached out and he slapped them as he skipped by.
See the verbs in there? Tension usually has lots of verbs, or so I've found. Even if it was a scene where they aren't dancing, there is one where Belome (the MC)'s brother corners him in a storage shed.
Belome had been talking to his brother about the upcoming festival and then recalls the last one, making a joke about it and laughing, wanting his brother to laugh with him. But his brother slowly walks over and shoves a very pointy stick in his face, one that he had been carving when Belome found him. His brother is angry that Belome took the main family job meant for the first born son. He shoves the stick up against his throat...
Transition from peacefulness to TENSIONS is important to make the tensions more prominent. Peacefull...peacefull...then gradually gradually colder and colder.
It's even better if the reader knew from previous parts the tensions between one character and the other. That way we can sort of be expecting something to happen but we are never sure when it will happen.
So, how to increase tensions?
Shorter sentences: and I forgot to mention but, sparser description. In tension scenes, detailed description becomes unimportant.
Verb it up, Scotty! (Beam me up, Scotty, Star Trek reference): Put lots of verbs in your sentences.
Transition is key: Peaceful...peaceful...colder...colder...OMG!
Happy Writing! :D