Verse 1: "Praise the Lord, O my soul; all my inmost being, praise his holy name."
When David uses the phrase "all my inmost being" he is saying "It's from the depths of my heart. I'm not faking it." He wants to be true and transparent in his praise to the Lord. It's the real deal. He's not going through the motions. He is passionate about his praise.
Psalm 33:21: "In him our hearts rejoice, for we trust in his holy name" (NIV).
Bill Bright says: "One of the greatest lessons I have ever learned is the importance of praise and thanksgiving. The greater the problem, the more difficult the circumstances, the more important it is to praise God at all times, to worship Him for His sovereignty, love, grace, power, wisdom and might; for the certainty that he will fight for us, demonstrating His supernatural resources in our behalf." (Promises, Day 155)
Verse 2: "Praise the Lord, O my soul, and forget not all his benefits-"
You will notice it was "all my inmost being" in verse 1. Now it's "all his benefits" in verse 2. Verse 1 is focusing on the internal. Verse 2 is focusing on the external.
Bless the Lord, O my soul, and forget not all His dealings with us.
Joni Eareckson Tada says: "A sacrifice of praise will always cost you something. It will be a difficult thing to do. It requires trading in our pride, our anger, and most valued of all, our human logic. We will be compelled to voice our words of praise firmly and precisely, even as our logic screams that God has no idea what he's doing. Most of the verses written about praise in God's Word were penned by men and women who faced crushing heartaches, injustice, treachery, slander, and scores of other intolerable situation."