Tips on Increasing Tension:
One technique to create tension is called "moment-by-moment." In this technique, you slip into the character's skin and feel every emotion, no matter how subtle, for a short period of time, such as one to two minutes—the period during which the tension or suspense needs to be heightened.
In your first draft of the scene, you write every emotion, every thought, every sensation (heart beats, prickling scalp, sweaty palms, hot, cold, etc.), smell, or sound that the character could possibly hear during that span of time.
Then you read it over and pick the most powerful emotions and sensations to keep. Delete the rest (or save in a workbook for future reference).
Another technique is to slow down time from the main character's point of view—like you're watching a film in slow motion or as if the main character, other characters he or she sees, and everything around him of her is moving underwater.
Then describe everything that the main character sees, hears, smells in this slow-motion pace. This should be used at a moment of high tension or suspense, and should be used sparingly. The length of the passage should be relatively short—a few sentences or a short paragraph.
The difference between "moment-by-moment" and "slow-mo" is subtle. In moment-by-moment, you're inside the character's skin, feeling and describing every sensation from the inside, and feeling and describing the character's reaction to outside forces, sounds, smells. Basically, you're describing what's going in inside the character. In slow-mo, you're describing what is going on outside the character from the character's point of view in a heightened emotional state. Both techniques can be combined to heighten the tension and suspense.
A third technique is kind of a mixture of both of these two techniques. You can achieve tension or suspense by slowing down the pace of a particular scene somewhat (but not as much as slow-mo). To do this, during the progress a suspenseful scene, you weave in commonplace sights, sounds, smells, or recurring scary thoughts in the main character's head.
A short line or two in between the scary parts draws out the tension contrasts the creepy with the normal, giving the scene an almost surreal feeling. It's also a good way to bring tension to a scene that would otherwise be a normal, every-day scene. For example, the main character is having dinner with a friend, the salad is being served, the smells of steak and potatoes fill the air, the normal sounds of cars passing and birds singing can be heard, and all the while she's hearing her ex-boyfriend's words in her head, If you leave me, I'll kill you, but she continues eating her salad, trying to control her trembling hand.