What should I do?
I've told you ten times already! (Present perfect because I told you in the past, and I am about to tell you again)

Where is that letter?
I have already posted it. (Present perfect, because the letter is still in the post, so the action is not yet complete.)

I did not receive your letter
I posted it ages ago. ('Ages ago' refers to the past, so use past tense)

I'm busy today!
What have you been doing? (Bill has used a present tense, so Sally wants to know what he did and is still doing, therefore use the present perfect continuous)

Your driving is improving
I've been taking lessons.(Sally uses present perfect continuous to show that she started taking lessons in the past, and has not yet finished taking the lessons)

You were happy last time I saw you
I'd heard some good news. ('Last time' is about a time in the past, so you need to use a past tense.)