How to develop a discipline of restrained speech...

Scripture warns about saying a lot and doing so rashly.

It associates “restrained words” with a “cool spirit.”

- “Whoever restrains his words has knowledge, and he who has a cool spirit is a man of understanding.” Pr 17:27

- “When words are many, transgression is not lacking, but whoever restrains his lips is prudent.” Pr 10:19

Conversely, it associates those who speak with “rash words” or “hasty words” with being a foolish and violent man.

- “There is one whose rash words are like sword thrusts, but the tongue of the wise brings healing.” Pr 12:18

- “Do you see a man who is hasty in his words? There is more hope for a fool than for him.” Pr 29:20

The unrestrained foolish speaker is presented as someone who is eager to share his opinions on all matters but they are rarely words that impart anything truly helpful.

- “A fool takes no pleasure in understanding, but only in expressing his opinion.” Pr 18:2

- “Leave the presence of a fool, for there you do not meet words of knowledge.” Pr 14:7

How do you develop this discipline?

Here are three actions (I'm sure there are others):

Pray

I've found that anxious and overly talkative people have a weak prayer life. Their verbosity is rooted in a need to work issues out loud. This can be temperamental to a degree. We all process things differently. Some do it externally through talking it out. But talk it out with God first in prayer. You'll find when you talk to others that you have less anxiety and tend to be more restrained.

Meditate

Take time to chew your food. Study an issue from multiple perspectives. Think about it for days, weeks, and months. You'll find that you are less inclined to confidently and rashly air your opinions and positions.

Listen

Listen to older and wiser men. Let them talk. Ask good questions. Pray and meditate on their words. You'll say less stupid things.