It's the same use with your question:
He is(was) going to talk a sentence.
他好好准备说一句话 is wrong use
You can say in these ways:
The test is difficult,You should PREPARE WELL.
I believe I can pass the exam with GOOD PREPARATION.
“是好吗”is not true,you shoule say:这样对吗？对吗？（means: is it right? right?）
I'll see you tomorrow,ALL RIGHT?
Will you send my breakfast up, PLEASE?
Did you sleep WELL?
It often used in a polite way.