Make your hero believable. Figure out the course of his life from the time he was born to the time your story starts, as if he were a real man. By knowing your hero so well, you include believable details. A hero is much more believable if you indicate he was raised by a single mother and worked at three jobs for four years to attend college, than if you merely indicate he is ambitious and well educated. Also, don't give your hero a big, athletic, muscular body without justifying how it got that way. He didn't spring up fully formed into a hero, after all.
Make your hero likable. He should not be perfect, but he should not be abusive or too domineering either, though it's fine for him to be angry and a little domineering. However, he should have traits that counter his flaws, such as bravery, passion, loyalty, faithfulness, intelligence, and all the things your reader would like in a real man.
Make your hero larger than life. This doesn't mean he should be able to leap tall buildings in a single bound, but he should be more admirable than any real man could be. While a real man may have doubts and insecurities, your hero may have plowed through life with utter confidence, overturning all obstacles and making a success of himself through sheer determination, with only the occasional moment of doubt that is easily overcome. No moving back home with his parents to live while he gets on his feet again for your hero! A romance novel hero can, and does, do things no real man can.
Make your hero desirable. Since mostly women read romance novels, your hero must be someone your female readers would envision being attracted to. He needn't be tall, dark and handsome--in fact, he can even be a little pudgy, with an average face. There are very few specific rules. But for mainstream romance, he must, no matter what, be attractive to the heroine and your reader. So even if he looks geeky, wears glasses, and doesn't have a lot of bulky muscles, he may be surprisingly strong for his appearance and possess a certain calm way of moving and sensitive hands that turn your heroine's bones to water.