A first step in helping your students to develop good pronunciation is to encourage them to believe they can do it. If, for example, your students are middle-aged and are learning English for the first time, they might have misgivings about their own ability to learn to speak a new language well. But, contrary to the outdated "critical period" hypothesis, human beings do not become physically or mentally unable to acquire a native accent in a new language. A person of any age has the mental and physical capacity to develop excellent pronunciation in a new language. Tell your students this, and encourage them to believe that slowly and patiently, they can learn to speak the sounds of this language well.
Every language has a limited phonology; a given language only uses a certain number of sounds. And depending on the phonology of the mother tongue spoken by students in your class, they may have more trouble with particular sounds in the language they are trying to learn. For example, if your students are Korean, they may have trouble learning to pronounce the "l" and "r" and "sh" sounds of English. If your students are American, they might have trouble learning to pronounce the throaty "r" and the nasal vowels of French. Identify particular pronunciation difficulties that your classroom has, and focus in on working on those specific sounds.
When it comes to pronunciation, the parts of the vocal apparatus (mostly the tongue and other parts of the mouth) are very important. If a student can come to recognize how he is physically forming a sound wrong in his mouth, he can then learn to pronounce that sound correctly in the future. So help your students to identify how they are mispronouncing trouble sounds. If a student consistently pronounces the English "r" as "l", show him how his tongue always touches the back of his teeth when he says "l." Show him how your tongue should curve to make the letter "r," and tell him that the tip of the tongue should not touch the teeth or upper palate at all. Working bit by bit through physical details like this can help students drastically improve their pronunciation.
Pronunciation is about more than just the sounds of the language. Good pronunciation requires the student to also have a solid grasp of suprasegmentals such as pitch, intonation, length and amplitude of an utterance. To help students develop good pronunciation, it is important for them to hear lots of language in context. It is also important to give them language as spoken by a variety of people. They should be exposed to various types of narratives (speeches, everyday conversation and arguments) being spoken by people with different accents of the language. Hearing many people speaking the language in context (whole discourses rather than just individual vocabulary words) will help students develop an ear -- and then a tongue -- for the suprasegmental features of the pronunciation of a language.