1. The reader must be able to relate to the protagonist in a positive way. Enforcing the reader's ego is the best way to get them to stick around.
2. Characters are channels for authors to preach their opinions. If the protagonist believes something, the author agrees with them. If the antagonist believes something, the author disagrees with them.
3. The only reason a reader should empathise with anything the antagonist says or feels is if the reader is a sick, twisted puppy.
4. Make your protagonist a blank template, so that readers can insert themselves in the character's place; an average Jane or Joe who also happens to be the most alluring biped in town.
5. Readers live vicariously through characters. Make sure your protagonist accomplishes amazing feats but still does "normal" things like go to work, crush on peers, and play sports.
6. The universally sympathetic protagonist is white, male, and heterosexual. Deviating from any or all of these factors will significantly marginalise your readership, as minorities are only relatable to other minorities, but majorities are empathetic to everyone.
7. Create an ensemble of characters, with different archetypal roles, so that you can tick off every possible template a subsequent reader might relate to.
8. Kill off your protagonist's family. This travesty will not only add to the reader's sympathy, but free your protagonist from familial restrictions.
9. Heighten the reader's emotions by allowing disturbing things to nearly happen to your protagonist. Don't let these traumatic events have any sustaining impact on the plot or characters, as you will depress the reader.
10. Once you have the reader in your grasp, inflict heartbreak and death-beckoning fates on your hero, even if it is overly angst-ridden or jarring to the plot. Using your characters as emotional weapons against your reader is the best way to ensure that your novel makes an impact.
Go forth and let your characters show readers how they should feel.