I'm working on a new novella at the moment and struggling with character development, pace, and all those good things. It's set in the ancient world, in and around the Mediterranean and researching life at that time was great. (I love the Eyewitness books for their great pictures and good overview of different time periods.)
But beyond the pace and the plot, what I am really trying to work on is improving how my characters come across and how the tension between them builds. Sexual tension in romances is crucial to how succesful the story is - it can't be too rushed, that is the number one thing to remember.
So in my novella hero and heroine meet in a crisis situation but they take notice of each other - nothing big, no long passages on how either of them look. It is a crisis situation after all but one or two sentences where they take note of something special about each other sets the tone or the foundation. From that initial meeting the chemistry between them builds and once they're out of the crisis and in a more relaxed setting they can begin to get to know each other better and savor the mounting feelings between them.
And them savoring those feelings is key - I've read a few stories where either the heroine is attracted to the hero but you don't really know why because the writer hasn't taken any time to explore the attraction beyond noting his bulging biceps or his piercing eyes or something. No, there has to be more than that. A lot more, at least for me. I like to have the heroine taking note of the hero's sense of humour, the way his veins cord his arms, how he fills out his trousers, how her touch affects him, etc., etc..