Samma Vaca, or Right Speech, is an important precept.
Maybe most of people here know it well, but its never to much to relearn or re-read such things again. Speaking well, using the right words, the right manners and choosing carefully what to say can be a great way to generate good karma and benefit other beings. The opposite is also true, thats why one must remember such important precept.
Briefly, the Right Speech is:
1- Do not lie; tell the truth.
2- Do not speak wrongly about others; when speaking about others, speak in such a way that it will create harmony, not division.
3- Dont use harsh words; be gentle while speaking.
4- Dont have idle chat; speak only when necessary.
Of course that this is the resume. Also note that Right Speech includes a negative part ("dont do this...") and a positive one ("speak like this or like that...").
Since i dont have the wisdow of a Buddha, i let Buddha himself teaching you more about how to use Right Speech. See the link below, because it covers many situations. When reading the article, one will feel fresh and new. I really enjoy reading the words of Buddha http://www.accesstoinsight.org/ptf/dhamma/sacca/sacca4/samma-vaca/index.html