Positive, wholesome speech has a great effect on others.
Proverbs15:1a
Proverbs 25:15
Ecclesiastes 10:12a

To avoid criticizing and gossiping about others, dwell on their virtues, not their shortcomings.
Philippians 4:8

The Lord blesses us when we do not speak badly of others.
Psalms 15:1,3
1 Peter 3:10

Avoid getting drawn into arguments.
Proverbs 17:14
Proverbs 26:4
Philippians 2:3

Try to not be offended when others speak badly of you.
Ecclesiastes 7:21-22
Psalms 119:165

Don't speak evil of them in return.
1 Peter 2:21-23
1 Peter 3:9

Let your good works disprove their lies.
Titus 2:7-8
1 Peter 2:12
1 Peter 2:15
1 Peter 3:16

Special chapter: James 3