You have to be the first reviewer of your essays. Ideally you should not stop rewriting an essay until you are comfortable enough to show it to someone else. While reviewing, pay attention to both content and style. While someone else might be in a position to comment on your style, only you can decide what is the best content for an essay. A reviewer will only be able to critique your content. He/she cannot create the content for you. Hence, for each essay, ask yourself if you have chosen the best story you have. Think if there is something else you can use to make the essay more effective. Creating a tight story is the first step. Look for glaring logical gaps in your essay. Systematically fill them up, till the essay is logically sound and convincing. If you do this incrementally each time you edit an essay, you will be well on your way to a perfect essay :-).
Partner in crime
For me, this reviewer was a person who was also applying to B-School (to the same ones as me in fact). But it takes a lot of trust to do that and I would say I just got lucky! Basically, you need someone who is familiar with the application process. He/she should be in the thick of things to have a reasonable chance of judging if your essays will sell. A fellow applicant is of course just an idea. A current student/recent alumnus can do the job too. But they are really busy and it's unreasonable to expect so much of time from them unless they are your friends. If you have a friend or colleague who really is interested in the process, or is ,say, applying next year, you have your ideal reviewer. You can coach him/her a bit with some sample essays of successful applicants. You can make him/her read school websites to understand what kind of people your target schools attract. Make sure you give them all the information they need to do this favor for you.
This is a person who does not know you very closely, someone who can give an outsider's take on your essays. This is the reviewer whose role is closest to that of an adcom member. It is difficult to get a stranger to review your essay for you. So get someone who is just a recent acquaintance/friend/colleague. You should be able to count on this person's goodwill and his/her language skills. All you need this person to do is to tell you if your story sounds like bullshit, did he/she find it plausible, convincing, impressive etc. Basically get an objective opinion about what did not go down well when he/she read your essays for the first time. If this reviewer has good lingustic skills, then he/she is the best person to candidly comment on your style, voice and language also.
This has to be a person who knows you really well. Family and close friends fit the bill well. They just need to check if the essay portrays the real you and if you have written in your real voice and tone. This reviewer should ensure that your true personality shines through your essays. Since this reviewer will be a person you are close to, you can take the liberty of troubling them a wee bit more and demand a little more of their time, sleep and effort. This review is the checkpoint for you to make sure that you are not trying to be the ideal B-School candidate, something that the adcom can see through very easily. This review should help you drop all your facades.
Human Spell Checker
As I said before, do not trust the spell checker in your word processor. You can do the spell check yourself. If you are not confident of your spellings, get someone who is good (even bright school kids will do, you can bribe them:P) to do it for you.
Do keep in mind that you need only the above roles in your reviewers. Having 5 unique reviewers is not very practical and is very time consuming. You will need atmost 2-3 reviewers with a reasonable overlap of the roles I have outlined. As the title says, they are angels,send them thank you notes, take them out for a drink or a dinner or something. Do not let your gratitude go unexpressed.
PS: This post is not a usual "how to review your essays" post because I did not intend it to be one. I believe there are only 3 things to review: Content, Style, Grammar. I feel that it is the selection of the right reviewers that gives you your best shot at submitting winning essays. Hence the slightly unconventional (maybe even tangential) post. Feel free to leave a comment for any clarifications you might need to find this post more useful.