I found this blog as a draft. I wonder why I never posted it...probably because it wasn't as complete as it could be. But I figured, it'd do no one any good just sitting there as a draft...so I'm posting it now. Enjoy!
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I wanted so desperately to do a word study on this verse because it is SO meaty.

Therefore, my beloved brothers, be steadfast, immovable, always abounding in the work of the Lord, knowing that in the Lord your labor is not in vain. (ESV)

So, my dear brothers and sisters, be strong and steady, always enthusiastic about the Lord's work, for you know that nothing you do for the Lord is ever useless. (NLT)

Therefore, my dear brothers, stand firm. Let nothing move you. Always give yourselves fully to the work of the Lord, because you know that your labor in the Lord is not in vain. (NIV)

Steadfast - from the Greek word meaning "settled."
Immovable - from the Greek word meaning "firmly persistent."
Abounding - from the Greek word meaning "to overflow or be in abundance of."
Labor - work, sorrow, trouble; literal Greek meaning "a beating."
Vain - empty or devoid of truth, containing nothing, purposeless

Practical little commentary from Pastor Chuck Smith:
"You can never say that about anything you do for the Lord. Your labor for the Lord is never in vain. Never in vain. You say, 'But, they didn't believe, or they didn't receive.' That's all right. God doesn't pay commissions, only salaries. He pays you for that labor that you did, not for the results of the labor. Just for the fact that you labored for Him. Therefore, abound in the work of the Lord, because it's never in vain."