The rule is easy enough with words that are always one way or the other. No one would forget to capitalize a person's name, such as John Smith, Sarah, or Jane. Likewise, no one would forget place names, such as Canada, Kentucky, or Topeka, Kansas.
It's also easy to know not to capitalize common nouns like dog, train, or camel.
The complications arise when there is a word that is sometimes capitalized and sometimes not, like river, king, or princess. Here's how to remember when a sometimes word gets the royal treatment. It all depends on usage.
If you are referring to that river over there, or any river, a river is a pretty common thing. It's a common noun and it gets no capital. If you are talking about a particular river, and the word river is part of the name of the river, then you use the capital letter. For example, you would write the river flowed past the campground, when it could be any river and any campground. In referencing a specific river, you would write In places, the Mississippi River is more than a mile wide. In this context, the word river is part of the name of the Mississippi River, and it gets its proper capital letter, because it is a proper noun.
The same rule-of-thumb applies to royalty and other titled personages. The princess crept quietly through the woods. However, Princess Tanella stopped for lunch. Likewise, a king can sit on his throne, but only King Fergasse can pass judgment on the people of Jurat.