The best way to learn about sentimentality is to read widely, both literature and pulp. Pay attention to your own reactions to books as you read, and study why they succeed or fail in provoking emotions in you. And for tips from one writer who deals with emotional subject matter with great skill, see Avoiding Sentimentality: Lessons from Eudora Welty, on her Pulitzer Prize-winning novel, "The Optimist's Daughter."
Finally, it's worth pointing out that it's possible to overcorrect for sentimentality, as John Irving reminds us in his New York Times essay, "In Defense of Sentimentality":
But as a writer it is cowardly to so fear sentimentality that one avoids it altogether. It is typical -- and forgivable -- among student writers to avoid being mush-minded by simply refusing to write about people, or by refusing to subject characters to emotional extremes. A short story about a four-course meal from the point of view of a fork will never be sentimental; it may never matter very much to us, either. A fear of contamination by soap opera haunts the educated writer -- and reader -- though we both forget that in the hands of a clod, "Madame Bovary" would have been perfect material for daytime television and a contemporary treatment of "The Brothers Karamazov" could be stuck with a campus setting.