Use capital letters for proper nouns. Avoid them for common nouns.
Proper nouns are the names of things. That means you use capitals for the names of people, places, months, days of the week, companies and so on. Don’t use capitals for common nouns.
People run into difficulty with capitals because there’s a temptation to use them for important words. In business writing people often use capitals as a way of avoiding offending someone or something by implying he or it isn’t important.
There is also the question of narcissistic capitals.
Some people think capitals adds authority to the words on a page. Nothing could be less true. Writing interesting, flowing prose is the way to win readers.
Another difficulty with capitals is when you write job titles. Newspapers typically use a capital letter when the title comes directly before a person’s name but not otherwise.
New Zealand Prime Minister John Key is correct, but it would be the prime minister’s desk. This even applies to the queen. No title gets capitals when it is not used as a proper noun.
In his book Newsman’s English British newspaper editor Harold Evans says;
“Avoid using them unnecessarily. The Parks Committee, but subsequently the committee. The South West Regional Hospital Board, but then the hospital board.”
One piece of advice I had early in my career as a journalist is: “If in doubt, use lower case unless it looks wrong”.
Lastly, do not use capital letters for emphasis and avoid writing words in all capitals. Barbaric companies insist their brand name is always in capitals.
There’s a good, logical reason why print publishers would limit capitals. As your eye travels along a line of text, those capitals distract you. It’s harder to immediately understand the copy.
This applies as much on-line as in traditional print.