Rules for Reading

I am currently reading a novel. The next book sitting on my shelf is Nathaniel Philbrick's Mayflower. I always read one work of fiction followed by one work of non-fiction. In fact I've done if for so long that I dislike stepping out of that system. If I am reading a novel and another novel comes along that I have really been wanting to read, it is a wrenching decision to pass on it because I'm supposed to read a non-fiction book next. My other habit is that in the mid-winter, after Christmas and into the quiet cold days of January and February, I always tackle some massive classic work that I've been meaning to read. In the past few years I've read Don Quixote, Ulysses, and The Tale of Genji.
I've read biographies before of people like Ben Franklin, Abe Lincoln and other famous self-taught men. They often have these systems set up for themselves, systems meant to keep them disciplined as much as to ensure that they are well read. In the end, I don't know if such a system detracts from the joy of reading, or if it enhances its value. I know I learn more when I make sure I read quality non-fiction between my readings of quality fiction.
Another thing I've often heard people discuss is how much of a book they will read before deciding they don't want to finish it. Some people read as few as ten pages, while some say that once they've begun a book they finish it, come hell or high water. There have been a few times I've stopped reading a book with only one chapter to go -- just too tired of the tepid prose or obvious plot twists or tedious 'observations.'
What do you think? Do you have a system for what you read? Do you have rules about what to read, or how much, or anything else?