There is no doubt that written communications matter. I recommend investing a proofreading service to ensure documents are thoroughly and comprehensively proofread to the highest standard (of course). However, not everyone has the financial resources or the desire to do this, so below are what I consider the most important tips for self-proofreading.
1. Don't try to proofread without taking a step away from what you've written first. No matter how careful and diligent you think you are, even under ideal circumstances you are more likely to miss errors in your own work than you are in that of others. Your brain will see what it expects to see, based on what you believe you have written, and this leaves you at an immediate disadvantage. So, proofreading while tired is definitely not a good idea. Even if you feel fine, go out for lunch, have a workout or sleep on it; in other words, take a proper break and come back to it when you are revitalised and able to concentrate.
2. Adopt a methodical approach. This is key to proofreading. The process can't be hurried, as every word, every punctuation mark and every sentence requires scrutiny. It is all too easy to skip over words, particularly the small ones, and this is when errors go undetected. With this in mind, proofread slowly and multiple times. Check for spelling first, then punctuation, then formatting etc.
3. Read it out loud. It may sound silly, but reading text aloud can highlight ambiguities and problems with sentence structure or readability that silent reading may not. If you can record yourself, this is even better. You can then play it back to find out if what you've written comes across in the way you intended. If you have someone who is happy to read it aloud to you, that's great too.
4. Make sure your surroundings are right. Keep the heating down, as a stuffy environment willl make you drowsy. Also, make sure you have adequate lighting so your eyes are not placed under undue pressure.
5. Make a list of things you are not sure of or need to check. Don't rely on your memory; it could let you down, and you could forget to look up that all-important spelling.
6. Don't be tempted to rush towards the end. If you feel yourself getting bored, frustrated or fed up, stop. Missing errors while on the home run because you have lost motivation will mean that all of your previous efforts will have gone to waste.
You may find different suggestions from other proofreaders, and the reality is there are many tools and techniques out there; what works for one person, professional or otherwise, may not work for another. But ensuring you take your time, have the right attitude and double-check everything will stand you in good stead to proofread your own work.