A) When it is stated "cover letter required", a hiring manager really means "a cover letter is required". This is not just to make you jump through hoops. A cover letter helps show numerous things. Firstly, one can mention relevant interests or experiences that relate directly to the type of job one is applying for, but which should not (for various reasons) be placed on the resume itself. It also shows that one knows how to follow directions. While I do not want to hire a drone, I am NOT hiring someone to just do what they want to do. They need to know how to follow directions. Finally, it is a chance to show off your writing and grammar skills. I know that not all people are very well versed in these matters; however, it is a chance to wow me with your erudition. As I work at a library company, being literate and well-written is always a bonus.
B) Job codes, like cover letters, are not things that are put in ads just for the hiring manager to see themselves in print. We receive a lot of resumes. If you don't put on the job code, it won't be read.
C) Personalize each resume and cover letter for each job. It doesn't have to be personalized much; however, one should make the effort of at least making sure the objective is, for example, relevant to the type of job you are looking to be hired into. Additionally, the objective of "Finding work" or "Looking to gain better understanding of self", while honest, is not an acceptable response.
D) Unless you have 10+ years of experience, you do not need to have multiple pages for your resumes. One page will suffice. Resumes are not an expression of self, but an expression of keywords and experience to attract potential employers to bring you in for an interview. Attempting to force employers to read five page resumes for someone who has only four years of menial office job experience will not get one in for an interview. It will get ones resume sent to the "no" pile.
E) While during an interview I will attempt to draw out of you what your hobbies and interests are, to get a fuller understanding of you, I do NOT care what they are on your resume, unless they are directly related to the job that you are applying for. For example, I have written "copy-edited for friends" on cover letters that are applying for editing jobs. I have not put that one for jobs that do not involve writing. Nor shall I do so.
F) Presentation is key. Let me repeat that - PRESENTATION IS KEY. I prefer all resumes to be sent to me in .pdf format, as that will ensure that any formatting that has been done, will remain exactly as one wished it to look like. However, I realize that not everyone has access to full adobe or uses Macs (where one can automatically create a .pdf file). I also realize that not everyone has access to MS Word or some other wordprocessing program. However, one should take advantage of whatever text tools they have, and create a clear, legible, and ONE PAGE resume to be sent to the prospective employer. We do not want to see:
o 16 pt font cover letters, with good portions of the cover letter either bolded, underlined, or both.
o Cool looking fonts, in 20 pt font, in the resume. Latitude is given to non-nationals obviously not used to US traditional resumes. There is, however, a limit to latitude and it shall not be extended to those who send me resumes in 20 pt, with Cursive fonts, take up three pages, and have not bothered to format it other than to use a "cool-looking" font.
o A "resume" that's 6 lines long, and obviously cut and pasted from another job email you had sent out previously.
G) Note that there's always a good chance robots are reading this for keywords. Keyword the hell out of this. It is better to have too many keywords, than too little.
H) Unless you are green or do not have a college degree, I do not care what High School you went to, what your GPA was, or what year you graduated. Remove it.
I) Repeat after me: "References are available upon request". References should not mar up your beautiful 1 page resume. They go on a separate page, with an explanation of who they are (friend, ex-coworker, ex-supervisor, etc) and they are not given to the prospective employer until asked.
Just follow these basic rules, and you too can have a legible looking resume that won't make a hiring manager glance at it, cringe, throw it in the rejection pile, and then use it for humor fodder while talking with colleagues.
ETA: To clarify, these are rules for US resumes. Resumes might be put together differently in your part of the world.