I've talked about this before but I'm coming back to it because it's of the great challenges of turning a piece of writing into a publishable manuscript and that's proofreading. Now there are basically three ways of approaching this; you can pay a professional to do it, you can get someone to do it for free, or you cant try to DIY.
Now I freely admit I have no experience of the first option for the simple reason that I haven't really had the cash to spare to do so, which I suspect is true of many aspiring authors. That being said if you can afford to spend a few pounds check out local writers groups or the various Google+ communities/forums that provide resources to writers and can help you make sure you are spending your money on the right person.
Option two may sound like a better bet; find someone whose willing to help out for free or maybe do a trade where you provide them with help on some aspect of their writing projects in exchange for a little proofreading. This has the potential to turn into a trap; if someone is prepared to volunteer to proof read your work then you can't really impose a clear cut schedule and time frame on what is after all a favour. This means that you are dealing with end dates like 'soon' and 'If I can find the time, maybe next week' and suddenly its months later and instead of a proof read manuscript you have an ex-friend or a family member who is no longer on speaking terms with you.
The simple truth is that most of your volunteers however enthusiastic they may be at the beginning of the process and insist that they know what they are doing really don't understand the tedium of literally making sure someone else has crossed every t and dotted every i and the process slowly grinds to a halt as the enthusiasm bleeds away. This doesn't necessarily make finding a volunteer a bad way to do things, especially if you can find someone to trade tasks with, but you need to be prepared for it to take a long time and involve a lot of prodding before its done.
Now you may by this point be thinking that the best solution is just do it yourself, you have plenty of motivation and you know how to write after all; so why not? BECAUSE IT WILL NOT WORK. I put that in capitals just to make it clear that this is a terrible idea. I've mentioned before that a common complaint about Kindle books is that they haven't been proofread properly and are riddled with typographical errors. I would be willing to place a small wager that if you could approach the author of one of those volumes they would swear that they had gone over every single line and correct every mistake in syntax, every spelling error and every missing word; and yet the errors are still there. This isn't because the author was sloppy or lazy; it's because they literally can't see the mistakes.
It's easy to illustrate the problem; your significant other gets their haircut and you don't notice for days or you can't find your keys only for them to turn up in plain sight on the table you had checked twice already. What's going on is that your brain is somewhat lazy when it comes to seeing; when dealing with the familiar it simply fills in details from memory rather than actually observing the world around you. Your brain knows the table is empty; it knows what your significant other's hair looks like so you see what you expect to see rather than what's actually there.
The same effect makes trying to proofread your own work an uphill struggle; you know what you meant to write and so more often than not that's what you see on the page rather than the errors and omissions that are actually there. One trick to get round this is to change the typeface and font size of your text if you are working on a computer but there really is no substitute for that second pair of eyes so really it comes down to either breaking out the cheque book or having a lot of patience and a talent for arm twisting.