When asking someone for something, think about what's in it for them. Demonstrate appreciation and gratitude. Positive feedback feels good. Thank everyone who does a task, or favor for you. Be genuine, specific, timely, and personal.
People become more likable by demonstrating appreciation and gratitude.
Keep your promises. Credibility and promise keeping are like Siamese twins, they are joined together. Credibility will never be established unless promises are kept. Promise only what can be delivered. Trust is established and maintained as promises are kept.