VOICING -- One thing that distinguishes one sound from other sounds is voicing. When a sound is voiced, the air used to make the sound causes the vocal cords to vibrate. You can tell if a sound is voiced by placing you hand tightly on your throat and saying the sound in isolation. If the sound is voiced, you will feel vibrations. (Try saying "zzzzz" with your hand tightly on your throat. Now try saying "sssss." You should feel the vibrations with "zzz" but not with "sss.") When a student is saying a sound incorrectly one reason may be that he or she is voicing an unvoiced sound or is not voicing a voiced sound.
MOUTH, TONGUE, AND LIP POSITION -- Another thing that makes a difference in sound is the position of the mouth, tongue, lips, etc. (sometimes these are called articulators). For vowels, differences in sounds are produced by how far forward or back the tip of the tongue is in the mouth, by how high or low it is, by how much the jaw is dropped, and by whether the lips are rounded or not. For consonants, differences in sound are produces by where the tongue touches in the mouth, by which part of the tongue touches, and by whether the air is stopped completely or partially and then released. With some consonants the differences are created with articulators besides the tongue (e.g. 'b' is produced by the upper and lower lip) or by the shape of the tongue rather than the position (e.g. 'r' and 'l' are different because the tongue curls up towards the roof of the mouth with 'r'). If your students are producing a sound incorrectly, try to imitate them to feel where your tongue is and how your mouth is shaped; them, move your tongue to the correct position to make the sound. You ought to be able to help your students get their tongues and mouths into the correct position in this way.
STRESS AND RHYTHM -- English has a stress and rhythm pattern that is different from most other languages in the world. Most languages give equal amount of time to each syllable. In English, we give an equal amount of time between stressed syllables. This means that we say many unstressed syllables very quickly. This difference is one that most ESL speakers will benefit from learning through lots of practice and examples.
INTONATION -- In English, there are two basic intonation patterns. With one, the speaker raises his or her voice on the last stressed syllable of the sentence and keeps the tone higher to the end of the sentence (called rising intonation). This pattern is used to indicate that what he or she is saying or asking requires a YES or NO answer. With the other pattern, the speaker raises the tone on the last stressed syllable of the sentence and then drops the tone to a point that is even lower than the tone before the voice was raised (called rising-falling intonation). This pattern is used for regular sentences and for most questions requiring the giving of some information as an answer. There are other patterns, but they are basically variations of the two described. You need to help students be aware of the intonation as often it can completely change the meaning of a sentence (e.g. "You're a high school graduate" can either be a statement or a question depending on whether it is said with rising intonation or rising-falling intonation).
Many textbooks may also give you pointers about English pronunciation. However, if they do not, you can usually figure out what the problems are by looking at the four aspects of sounds that were discussed, namely, voicing, position, stress and rhythm, and intonation.