Written Communication is a very sensitive area with most dyslexics, for it is the area the most people will see and criticize us about. It is also an area that is so very important that we over come. For there are very few jobs that do not require some form of written communication. Even if it as simple as leaving a note for someone.
For 'long-hand' writing (pencil and paper, not using computers) I do not have any good suggestions, other than take your time and cover each preceding line as you are writing. While one should be concerned with spelling, do not make it your primary concern while trying to get a paper finished in a set amount of time, in my opinion the content of your work is more important. As it is unlikely a person with dyslexia will get all the words spelled correctly in a complex paper without taking far more time than can be realistically allowed.
Thanks to the invention of desk top computers and spell checking software, plus the fact that most businesses & homes have these systems and printers. Many of our common mistakes can be corrected (at least enough to make our writing presentable). Anyone with dyslexia should ALWAYS spell check any document before sending it out (even if it is just a note).
When typing, I will set the print size to a very large font . This makes it easier for me to read my work on the terminal, in turn, allowing me to write a little more effectively.
After I have completed typing the work, the spell checker always gets a very hard work out. The spell checker is probably the tool that has helped me to learn the spelling of words more than anything else. As it simply shows the errors and a possible corrections without additional comments, frowns, glares, grunts, head shaking, etc.
After I have the document the way I want it to read. I simply change the fonts to a smaller print size, for everyone else, and send the information out.
By no means does this make my writing perfect, however, using the common tools that are available, and a lot of practice, does make writing much easier and more effective.
While we are always going to make spelling and grammar mistakes, it does not mean we should not try to do better.