After seeing countless application packages and talking to a fair number of hiring managers, the one thing that applicants tend to need the most improvement on are their cover letters. As with anything that will be reviewed by a human, everyone will have their own things they want to see in an application package, so take the following with a grain of salt. Some people want to see a thoughtful cover letter accompany your materials and others could care less what it says, it all depends on the audience. With that said, here are some tips on writing your cover letter:

• It's not a copy of your resume. Don't waste your word count on repeating what your resume already communicates. Too many people write cover letters that say things like "In the summer of 2012, I worked at x firm and did x things, and then I worked at x and did x and then..." See what I mean? 
• How does your experience fulfill the need of your prospective employer? A better approach when mentioning past positions is to talk about how those experiences equipped you to fulfill the role you are writing about. For example, "During my time working on the modernization of Westinghouse High School with X Firm, I was really able to solidify my understanding of the nuances and intricacies of the DSA plan check process..."
• What have you done that displays your alignment with your prospective employer's mission and core values? Most (good) firms have some kind of focus or mission, that might be an emphasis on education projects, or high-end residential, or healthcare. Whatever it is, if you are able to connect something you've done in the past that shows your alignment with that focus, it can pack a powerful punch. Maybe you're just starting out and you really want to work on towers, then (briefly) share how your thesis project from school explored tower design. Or perhaps, you have an interest in more social and community-based projects, then talk briefly of your past volunteer work in the community and how that ties into the work of the firm.
• Don't over-sell. A common mistake is saying things like "I think I am the best candidate for this job," or even "I really want to work at your office." Unless you've been to the office before or personally know people who work there, you don't really know that, and neither does the employer, that's the point of the interview. A better approach: "I really admire the work the firm has been producing and I'd love to learn more about this opportunity to see if we might be a good fit for one another." 
• Referencing a recent project can work wonders. This is a bit harder to do, but if you can mention one of the firm's recent projects and how you were able to identify with their mission and values through that project, it can potentially make a strong impact. You might say something like, "I really appreciated the firm's recently completed Gilroy Residence, the home's craftsmanship and use of natural materials clearly embodies the studio's belief in the architect's role as 'master-builder.' It's this holistic approach to architecture that I myself have a deep passion for."
• Keep it brief. No more than a page, preferably shorter. Remember, you're only expressing your interest in the open position and concisely making a case for why the prospective employer should take the time to learn more about you. A good closing to your letter could be something like: "If my aptitude seems to align with what the team is currently seeking, I'd love to schedule a time to meet..."

Those are just a couple of pointers. As mentioned above, take them all with a grain of salt, there are many wrong ways to do something like a cover letter, but there are also many right ways to do it. In the end, you need to know your audience and utilize that knowledge to communicate why you should be considered. Good luck, now go apply to your new job!