The five senses are vital to how we as humans experience the world around us. Sight and sound get the most attention because they're more vital to our physical well-being (sight allows us to maneuver through our obstacle-filled world, sound gives us warnings and helps us communicate). It's no surprise, therefore, that they're the senses that we instinctively try to describe in our books. However, the other three senses can be vital to drawing your reader fully into the world you're creating in your story.
Did you know that your sense of smell is extremely important to recalling memories? The sense of smell evokes memories in a way that no other senses do. The sense of smell is also key to how we process our environment--we can actually smell fear, though we don't necessarily process it consciously. The sense of smell can play an important role in your story, whether it's the scent of a lover evoking a memory, the acrid odor of smoke giving warning of danger, or the smell of home cooking whetting an appetite--and your description of those smells will bring about a similar reaction in the reader, drawing her deeper into your story.
For more information about the sense of smell, this article is chock full of scientific information that could provide fodder for ways you could use the sense of smell in your stories.
Touch is especially important in romances because touch is vital to intimacy. The obvious use of touch is in love scenes, where touch is key to building sexual closeness and giving pleasure. But touch has other uses in fiction. Where, when, how and how often characters touch each other helps convey to a reader whether or not the relationship is advancing or retreating. People in harmony touch each other almost without thinking. For people in conflict, however, touch is like a loaded weapon. They weigh the consequences of touch--will he read the wrong message into it? Will it bring us closer? Will I be able to stop touching him even though I don't trust him? Will my touch make him as vulnerable to me as I am to him?
Characters who have been in harmony--and therefore touching each other often--may stop touching when they are hiding things from each other, out of fear that they may convey by touch the secrets they're trying to keep from one another. Use the sense of touch to give your readers cues to the emotional state of the characters and their relationship with one another.
Sometimes I think taste is the hardest sense to write, for a couple of reasons. First, people usually can't choose what they see, hear or smell, and sometimes they can't help what they feel with their sense of touch. But generally, people can choose what they taste, and they can be very picky about it! For instance, I love salsa, mushrooms and sweet red peppers. My best friend Jenn won't eat any of those, and no matter how beautifully I describe the way those things taste to me, she's not going to be impressed. So I can find it intimidating to write about tastes. Taste is so subjective.
Second, taste is the sense that is most intricately intertwined with other senses. For instance, have you ever eaten a food that you didn't like because you didn't like the texture of it in your mouth? My friend Jenn doesn't like mushrooms because of how they feel on her tongue. And how many people won't eat a crawfish because of how it looks on the plate? Plus, taste is particularly bound to smell; scientists believe that 75% of what we call taste is actually smell. (Have you ever noticed that when you have a bad head cold, and your nose is stuffed up, your food doesn't taste as good? That's why).
However, taste can bring a lot of texture to your story. When I describe the "chalky, fake-orange" taste of antacids my character is chewing, you can imagine it. You can understand his grimace of distaste, get a sense of just how churned up his stomach must be for him to be eating antacids like candy. If I have my heroine becoming nearly orgasmic over the rich, sweet-tart taste of a chocolate-covered strawberry, you can imagine why the hero's eyes glaze over and his jeans feel a little too tight as he watches her eat it.
So remember, when you're writing your story, try to engage all of your reader's senses. It's the best way to suck her into the world of your story and the lives of your characters.