A great place to start is by looking at sample cover letters. The Sample Cover Letters website I've linked in this post is a helpful resource whether you're just sitting down to write your first-ever cover letter, or you're looking to improve upon the letter you've been using. There are lots of relevant tips on basics like how to format your letter, how to make your paragraphs flow smoothly, and how to emphasize your strengths and explain any issues such as gaps in employment.
Another cool thing about this particular website is that they have samples relevant to specific jobs including teaching, dental assisting, and medical assisting. And if you're an entrepreneur, here's one other interesting point: there is also a sample business plan cover letter that you can use if you're writing a cover letter to a potential investor, bank, or other business-related institution that might be helping you with funding for your business.
Did you know that a lot of job applicants don't even bother to include a cover letter when they apply for a job? You can set yourself apart from the crowd just by taking the time to compose a well-written letter. And with detailed samples to give you a solid starting point, there's really no excuse to not have a great cover letter as part of your job hunting portfolio.