Omniscient point of view is, in some ways, a complete lack of rules. You tell the story essentially from the point of view of God. You can dig into the thoughts, dreams and wishes of any and every character in your story. In fact, that is the big advantage of taking the omniscient viewpoint: revealing the thoughts and opinions of multiple characters in a single scene. You cannot do this with limited viewpoint. If you'd like to include a fair amount of dramatic irony in your story, omniscient viewpoint is a good way to achieve it.
Limited point of view is the telling of a story through one character's viewpoint. Readers are guided through the story by this single character. Third-person, limited viewpoint is the most common way to tell stories in fiction. Limited viewpoints allow readers to intimately get to know a character. The only better viewpoint for character intimacy and immersion is first person. However, writers have to write the entire story in this character's voice when writing in first person. In third person, limited viewpoint, writers can use more of their own voice -- while achieving an almost equal amount of character immersion.
You are allowed to change viewpoints when writing with the limited point of view. However, you cannot switch viewpoints at random. You must signal to the reader that a change has occurred. The most common way to do this is a chapter break. Another way to do this, is by placing three asterisks in the center of the page on their own line. Changing between viewpoints also allows you to tell the story of one event from multiple points of view. This can create tension, as you reveal more truths about an event.
Reading a book written in a way that you'd like to write is a fairly obvious tip. However, you shouldn't overlook how powerful it can be. Pay close attention to the way the author uses viewpoint. You should identify whether the story is omniscient or limited, as soon as possible. Pay attention to the way the author uses the viewpoint to enhance the story. Aside from the conscious, you will somewhat subconsciously mimic the writing style of what you're reading without even trying, which is the main benefit of reading a book with a style similar to one you'd like to write in.
If you're writing fiction, you'll probably write in past tense just fine, without even trying. If you are someone who slips out of past tense when writing, the best thing you can do is practice. Also, acknowledge that -- just because you're mainly writing in the past tense -- doesn't mean that you can never write a present or future tense sentence. You may simply be over thinking it. Also, try reading your work aloud slowly. This makes it easier to identify your own errors.