Do you know what drives me crazy? People who write things like "your stupid" when posting comments on internet forums, youtube, digg, and anywhere else that has a comment box. As a lover of the English language, it pains me to see minor atrocities of this sort on a daily basis. This instructable is not meant as a panacea for the internet's grammatical difficulties, and as such it is by no means comprehensive, but every little bit helps.
If I am legitimately wrong on something, let me know and I will make every effort to correct my mistake. In such instances, I would appreciate it if you would quote me chapter and verse of The Bedford Handbook (6th Edition), or at least give me the error code; this is not necessary, but it would lay my mind to rest.
Again, this is by no means comprehensive, so cut me some slack.
Step 1: The "Shift" Key is Your Friend
One of the things that bugs me is when I see sentences that do not start with a capital letter. Seriously, there is no good reason not to start your sentences with capital letters, and people won't immediately lose a little bit of respect for whatever it is you have to say if you use capital letters properly. If for some reason both of the shift keys on your keyboard are broken, the best thing you can do is to abandon the keyboard immediately; it is no good to you anymore. As a stopgap, you can use the "Caps Lock" key, but please remember to disable caps lock after the first letter, writing in "ALL CAPITALS" is just as bad as never using capital letters at all.
Another important thing to remember is that proper names and the first-person pronoun are capitalized; I do not care what Paris Hilton says, proper grammar is important.