If you decide to vouch for someone, you have to follow a few rules.

1- Don't oversell
Hedge yourself; you never know when people will disappoint you. Even the few people you will vouch for might one day slip. Overselling some guy only sets you up for a steeper fall from grace.

2- Keep expectations low
Don't guarantee the moon. No matter how great the person you vouch for is, all you need to let the other person know is that the person you're vouching for can get the job done, nothing more. If there is an upside, you look even better. You'll get the rep of being both prudent and intelligent. No harm there, is there Shirley Temple?

3- Use gentle persuasion
Supply the person you are vouching for with what I call gentle persuasion, i.e. "you better not make me look bad, or I'll clip your sorry arse" kind of persuasion. In case they decide now would be a good time to let you down, let them know that there will be consequences.

No matter what you do, be smart enough to realize who your audience is when you vouch. The kind of person you are vouching to (the person who will hold you responsible) will affect your threshold of "vouching."

If the person on the other side is the local garbage man, you might have a little more leeway to vouch for every bum in your neighborhood. If you need to vouch to Silvio Berlusconi, well let's just say your standards are going be a little higher.

And when in doubt, don't vouch at all.

Watch your backs and keep your noses clean

From www.askmen.com