God's Will--A question

K. So I got a few questions for you guys:

1. A friend threw me at this the other day, and its still in my head bugging me (cough, cough--BEN!). It's a question that has to do with God's will. The Bible says about God,

"I know that You can do all things, And that no purpose of Yours can be thwarted."
(Job 42:2)

Here's my question--if God's will can't be thwarted anyway, then why do we pray for his will to be done?
If you got an answer for this, please post it up here, and have Scripture ready to back it up, (one rule--NO SCRIPTURE TWISTING! GOD IS WATCHING YOU!!! ).