There is some good advice above, but I would add a couple of things from my own experience as a recruiter...
ALWAYS present yourself professionally, whether on paper or in person (a chap arrived at our offices recently to hand in his CV; he was wearing a hoodie and jeans).
If you aren't confident in your writing and spelling, get help... Too many CVs and covering letters contain errors. This conveys the wrong impression.
DO target your 'marketing'... A scatter-gun approach may be worth considering if you have time on your hands, but otherwise, it's a very good idea to research the jobs you most want, and then systematically present yourself as a quality candidate, to the people who make the decisions at each stage of the process...