I didn't write the best recommendation letters as an ensign, but I got smarter by the time I pinned on O-3 and have done a lot better. Out of the last two Sailors I wrote letters for, one got at least one of the scholarships he applied for and another got an appointment to Officer Candidate School, beating the abysmally low 16% selection rate this year. Here are some pointers so that you don't make the same mistakes I did.
1. Do not write letters for mediocre Sailors. While it may be an honor to be asked to write a letter, it's also not a given. The CO doesn't sign every recommendation letter, and you shouldn't recommend just any Sailor that asks for your recommendation. If you wouldn't write a "Must Promote" or "Early Promote" evaluation for the Sailor, don't bother writing a letter. Kindly refuse and recommend the Sailor talk to someone else.
Why do this? Letters are cheap, so why not write a ton? A few reasons. For starters, your time is important. If you spend your time writing mediocre letters for mediocre Sailors, you'll have little time to write awesome letters for the really good Sailors. Most importantly, you need to think about what sort of message you send your Sailors when you just recommend anyone that comes in. If you are discerning in who you recommend, it'll inspire Sailors to work harder.
2. Never send a boiler plate letter. I've been asked "Don't you officers have a default letter of recommendation sitting around?" The only correct answer to that question is a resounding "NO!" NEVER send a boiler plate letter. Ever. They are a waste of space. Having sat on recommendation boards before, boiler plate letters, with their fluffy language and meaningless words, are promptly thrown out and forgotten, or worse, lead us to think that the candidate wasn't all that much qualified, since the person didn't make an effort to write the letter well. On the same thread...
3. You write the letter, not the candidate. This goes against a lot of common logic, since you probably write your own awards and FITREPs. But letters are different. Unless you're an admiral, you should write your own letters. If you follow rule #1, you won't be writing a lot of letters, so it'll be much easier to spend the time on them. Writing them yourself also gives you good writing practice, an important skill if you want to make real change in the world.
4. The candidate owes you a few items: a list of their achievements, who the letter is going to, and what they are applying for. Even if you worked with someone, you likely don't know everything that is going on in their life. A list of achievements will help augment what you already know about the person and make adding facts easier. Who and what the letter is for is important because...
5. Every letter is customized for the audience and desired effect. Read about the program you are recommending the Sailor for. If it's an award or scholarship, look at who won it in the past (likely on a website). Read their writeups. See what sort of candidates that scholarship or program selects. This information helps you pick what pieces to highlight in your Sailor.
6. Letters are one page, signed, and with contact information unless the program says otherwise. People openly sigh when they see a staple, so don't bother with a two page letter. Cut it down to one page, which will make you cut out fluffy language and passive sentences. Put your signature on there (digital or hard copy) and contact information (address, phone and email). This ensures that should the selection personnel be skeptical of the authenticity, they can call you and ask about the letter.
7. Numbers and cause/effect rule the day, but ensure it doesn't sound like a machine. Saying a person is "nice" doesn't help. You need some actual numbers and data in a letter, and especially cause and effect relationships. However, there is a bit of room for how you feel about them. In a one page letter, give yourself 2-3 short sentences to say something subjective. I've used phrases like:
"I trusted Petty Officer X with the lives of many other sailors, including my own, and was never disappointed with the results. By selecting him to join the ranks of your other distinguished members, you will only continue to raise the prestige of the XX Honour Society."
"His professionalism inspires both his subordinates, peers, and even his officer supervisors. It certainly inspired me."
Having just a small bit of emotion adds the right touch to a numbers and cause/effect heavy letter. It lets your audience know you care both about performance and about people.
8. Be on time. Should go without saying. Get your letter off in a timely fashion. A late letter looks terrible, and may even be ignored.
