Keep the pace fairly quick, followed by slower ones. If you add an action scene followed by a slow scene, the action scenes will be more suspenseful, than if you just add action after action.

Don’t give the reader all of the action at once, keep it flowing with the story, make unsuspected turns, make the reader think something is going to happen, only to find it was a false lead. (Like in a horror film, when you think something is going to happen, but something else happens)

The aim of suspense is to get the reader wanting to know more; what’s going to happen, or who done it? If it’s a who done it story, don’t reveal until the end and try getting the reader to believe somebody else did it. Do this by misleading, characters lying ect…

Another good way is adding small cliffhangers after each chapter, mostly done in novels to add the suspense feel, but still a good way.

And probably the most important is show, don’t tell. Doing this will add more depth, if we can see what’s happening to the character, then that’ll create a picture and create suspense on its own. For example don’t just say he was scared, show us how he was scared, shivering with wide eyes, heart beating faster with sweats, eyes fixed on the spot, he froze.
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Example:

Bob ran down the hall to the open door, bodied, dead bloody bodies filled the room.

Or

Bob ignored the deep stabbing pain in his chest and kept running. Please God don;t let me be to late he prayed as he barreled though the open doors. His heart sank. Bodies, dead mutilated bodies littered the floor. His will, his resolve leached out of him, he had been to late.
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