One of the most influential books I've read is "How To Win Friends And Influence People".
I was reserved about picking up the book, because it sounds quite manipulative...
Needless to say it's not. In-fact, it's quite the opposite.
One of the key lessons I learned is how to deliver criticism to people, because I know the most influential things people have said to me are rooted in the truth - and them being brave enough to deliver criticism.
Here are the key principles it shares:
Principle 1: Begin with praise and honest appreciation (always begin the conversation in a nice way, with genuine praise of their hard work and effort)
Note: Don't follow praise with a "but", that's not genuine praise.
Principle 2: Call attention to people’s mistakes indirectly (allowing them to come to the conclusion themselves, giving them pride of solving their own issue)
Principle 3: Talk about your own mistakes before criticising the other person (because we all make them, and it softens any advise that follows)
Principle 4: Ask questions instead of giving direct orders (once again allowing them to come to the conclusion themselves, giving them pride of solving their own issues)
Principle 5: Let the other person save face (damage your own ego before you damage theirs in public, take a blow if it means they save face - then apply the other principles in private to provide support or criticism)
Note: Taking the blow yourself is a sign of strength, not a sign of weakness.
Principle 6: Praise the slightest improvement, and praise every improvement (They quote: "Be hearty in your approbation, and be lavish in your praise")
Principle 7: Give the other person a fine reputation to live up to (speak of them highly, and big them up to someone to live up to)
Principle 8: Give encouragement and make faults seem easy to correct (which gives them the confidence to have conviction)
Principle 9: If working with someone, or if they work for you, or they are helping you, make them happy to do what they're doing (give them a part to play [a leadership role or something they own] and make the work they do meaningful)
Offer criticism kindly, and without damaging their ego.
Someone may be trying their best, or have worked extremely hard to help you, or they're struggling, or they have a big ego. Hence why how you frame criticism is important. Because if you approach it poorly, should you be close with that individual they will resent you, and if you work with them they won't want to work with you - which means you shoot yourself in the foot whilst you're at it.
Offer practical advise in the form of a suggestion.
Having said this, I would recommend reading the book for the full context.