The First Person
As much as possible, avoid using the first person pronoun (I, me, myself, mine, etc.) in your resume. While you may refer to yourself in the first person within a cover letter, it stands out rather inconguously in a resume. Instead, hack down your sentences and present it in the form of short bulleted points. They seem more professional and add more value to your statements.
The Tense Factor
Also be very sure of your use of past, present and future tense in your resume and cover letter. As you generally lay down previous achievements on your resume, it is advisable to stick to past tense. For example, "Enhanced productivity in sales management by 23 percent." You can be a bit more lenient in your cover letter. But in general, stick to the tense you started with throughout the letter.
Effect/Affect
Affect is a verb, and effect is a noun. Affect is an act/process that brings about a change. Effect is the resultant change itself. Be sure you do not mix these up. You may say, "The industry was affected by the recession. The effect was a series of lay offs." If all else fails, remember: Affect is an Action; Effect is an End Result.
Commas and Semicolons
Commas and semicolons usually get strewn around at random in your cover letter. However, they usually serve two specific purposes. The comma is used to separate part of a sentence which cannot exist separately. For example, "For my pioneering work in molecular biology, I was awarded a medal." Semicolons are used to connect two parts, which can be separate sentences by themselves, which add meaning to each other. "I did some pioneering work in molecular biology; I was awarded a medal for my efforts."
Your, You're, Yours, Your's
It's amazing how many of us get this wrong. Your is a possisive pronoun, signifying somthing belongs to you. You're is a contraction of you are. It is merely an abbreviated form. Yours is again a possessive pronoun, used in the passive form. This is the valedicotry form used to close a letter (as in "Yours truly"). The word your's simply doesn't exist in English language.