Not only do unintended mistakes in your final copy undermine your credibility, making the effort look hurried and amateurish, it also gives a poor impression of you having put in any effort to deliver a professional writing job.
This only lends further weight to the general notion of freelance writers not being serious enough about their work! Now, that's a tag we are all working to shake off (along with our bunny slippers!).
But, as real-time freelance writing professionals, we feel we work just as hard, if not harder, than our on-payroll pals at the traditional and new media offices worldwide...
so we really shouldn't leave any room for counteracting the efforts we have put in to our freelance writing work by slipping up on great copy due to avoidable proofreading mistakes, don't you think? Here are the top 3 proofreading tips to help you realize your writing success dream sooner than most:

1. Keep an eye out for the common errors.
   Make a list of the most common mistakes and keep this handy to refer to so you know what to look out for when proofreading a text.
   For example, words like affect and effect, their and there, your and you're, for and of, to and 'ot' and similar spelling and typo errors.
   
2. If you have a grammar or language checking tool on your system, remember to switch over its use for the client's preferred style of writing or in-house guideline, such as US or UK English, which often varies in spelling and cultural aspects of language on catch phrases, idiomatic usage and meanings.
   You don't want to be writing for a US audience using British phrases or vice versa because your content will lack the impact of custom created text.
   So, watch your colour and color, look out specifically for turn of phrases that are colloquially used if writing a location specific article.
   
3. Bring in a fresh set of eyes to read your final copy.
   Take the help of a family member or friend for proofing your completed work so a more discerning eye than the one that created the text gets a chance to look over for mistakes that your eyes would skim over without realizing these are errors, since it is natural for the writer to become familiar with the written text and thus, read it as a flow instead of in parts.
   Your proofreading assistant will read the same text, minus the disadvantage of familiarity with content and thus, be able to point out mistakes that you could have inadvertently allowed in the final copy.