It's the same use with your question:
He is(was) going to talk a sentence.
他正准备说一句话。
他好好准备说一句话 is wrong use
You can say in these ways:
那场考试很难,你要好好准备。
The test is difficult,You should PREPARE WELL.
我相信好好准备这场考试我就能通过
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?
我明天见你,好吗?
Will you send my breakfast up, PLEASE?
请你把我的早饭送上来,好吗?
Did you sleep WELL?
你睡得好吗?
It often used in a polite way.