It is always useful to send a covering letter with your CV, to explain some obvious questions like why you are sending the CV, what particular job interests you, what attracts you to the company etc. Take the time to provide some background information as to why you are applying and it will help to put your CV into context.
When sending the covering letter, make sure you have already identified the correct person to send it to i.e. establish their name and then address it straight to them. If this information is not readily available on any advertisement, make a call to the organisation. This shows that you have gone the extra mile.
In terms of the letter's presentation, it needs to be absolutely free from errors e.g. typos. Few people are good at checking their own written work for typos because we see what we are expecting to read; others are more likely to find your mistakes, so make sure you run it by someone before you send it off. Have it laid out correctly with the job title at the top and any relevant information such as a reference number. A typed letter is more professional, but if you have to write it, make sure it is neat and tidy. One page is always sufficient for a cover letter - any more and you risk it not being read. Never start the letter with 'I am writing...' - they already know that. Use paragraphs and ensure sentences are not too long, wordy and tedious.
You will want to give the impression that this is the one job/role you are really interested in. Make the letter as tailored and bespoke to the role as possible, so that it does not look like you have posted your CV off to 50 different organisations. If you have addressed it to an individual, write it as such. Also avoid generalisations. The person will want to read your letter as though it was specifically written for them.
Use the advert to assess how formal/informal the letter needs to be. You can tell from how it is worded as to whether it is conversational or formal and script your letter accordingly in order to match the tone and style. Bring specific reference to your main skills, abilities and experience into play that relates directly to the role, so that the reader is encouraged to give your CV proper attention. If there is nothing in your letter that suggests you are a good fit with the role, then your CV may not get read at all.
The first paragraph should set the scene by explaining why you are writing to the person concerned, highlighting the position of interest and why you are applying. The next paragraph is where you sell yourself and explain why you are the right person for the job, promoting your strengths that align to the needs of the role. Mention any relevant achievements here too.
If you can mention specific products or services that the organisation makes/sells that impresses you and has helped draw you towards this company, then carefully work it into your letter e.g. "I have always been impressed with the high standard of service I have received on the Virgin Atlantic flights and I would be proud to be associated with such a team."
Be mindful of deadlines and any mention of interview dates or assessment centres - you may see this information in advance on the advert. If there is a block of possible dates, outline any that you know you are unavailable for as it will help with their planning.
At the end of your letter, tell the reader what you propose to do i.e. call them, await their call, await their response etc. Put the ball in their court so that they know what is required to meet your expectations. Finally, thank them for taking the time to consider your application/CV.