Tense changes in fiction writing are a sign that you're a newcomer as an author. If your story's tenses fluctuate, you can discover any such inconsistency with careful editing, or by re-reading what you've written. Editing is simplified by the fact that the most consistent, readable and unobtrusive tense for fiction writers is past tense.
Why Past Tense Works
Consider the sentence: "He turned and looked at her; his hands trembled with fury." We are obviously at an exciting and suspenseful moment in the narrative; a murder, a scuffle or at least an angry confrontation is about to take place. The past tense voice of the narrator is virtually undetectable here; he as storyteller is not placing himself in the action, but, like all good tale-tellers, he keeps out of the picture so we can read what happens. If you change the tense of this sentence to present or future, your narrator's voice immediately intrudes.
Why Present Tense Doesn't Work Well
Take the suspenseful moment and make it present tense: "He turns and looks at her; his hands tremble with fury." Unless this is a synopsis letter for an editor, it is both obtrusive and awkward. Even masterfully handled, as in Cormac McCarthy's action scenes in "Blood Meridian," the use of the present tense calls attention to itself; it presumes the action happens in front of narrator and reader, both of whom are now in the story. This works only if the entire work is present tense, a difficult write at best.
Why Future Tense Doesn't Work Well
Future tense is even more problematic in writing effectively: "He will turn and look at her; his hands will tremble with fury." Unless your speaker is a soothsayer or an extremely clever fellow who has set up these two murderous pawns, the sentence simply doesn't work. If a mastermind planner is outlining a crime scene for secondary characters, it's a little better as a sentence, but still far from believable or realistic. How, for example, could anyone be so foresighted as to predict that a man's hands WILL tremble with rage?
Stay Consistent to Be Consistent
Stephen King, in "On Writing," recommends present tense only for very short fiction; he gives no credibility to future tense usage. In determining how to avoid tense changes in writing fiction, you would do well to follow the master of suspense novelists and stay only in the invisible and unobtrusive past. Stories, which were originally passed down orally, were all told that way; keeping your story in a single, past tense will also remove the temptation to time-travel it all over the narrative cosmos.