Regardless of how long you have been writing essays for school, writing is a process and there is a good chance that you will never be an expert. Even the best writers are guilty of making a mistake from time to time. Fortunately, if you know what to look for, you can weed out the most common mistakes when you proofread.
For a number of assignments, particularly those require research or factual information, you will need to cite the sources that you have used for your information. Instead of saving this for the last minute, label each fact that you use with a number that indicates its source. Make the reference page essay at this time and insert them in alphabetical order as well. This will keep you from scrambling to throw a reference page together and insert in-text citations at the last minute.
No student likes the extra time and work that they have to put into drafting an outline before drafting their essay. Outlines, however, serve two purposes in the writing process. First, outlines give you a chance to organize your ideas. If you cannot get them down on paper, try writing your main points on notecards and physically arranging them. The second purpose that outlines serve is making it easier to draft your paper. After all, once the main points have been written down you just have to add introductory and transitional phrases to make everything flow together.
If you do find that you have to do your own proofreading, set your paper down for a few hours before you proofread it. When you are ready, read through it twice. The first time, check for readability and the flow of the assignment. The second time, check for spelling, grammar, and punctuation errors.
Writing is a process and you should not expect to craft the perfect essay every time. This becomes increasingly true as you juggle more responsibilities and classes, which is likely to happen as you advance in your school career.