This Psalm tells us three things. First of all, we are to praise God for who He is, "his excellent greatness". Indeed our God is great beyond our wildest imaginations. He is great in His character. His love, mercy and grace are so great! He is great in all His attributes such as His omnipresence (everywhere at all times), His omniscience (all knowing) and His omnipotence (sovereign).
Secondly we are told to praise Him for "his mighty acts". We certainly do praise Him for His mighty acts in creation but above all else we praise Him for His mighty act in the new creation, creating a new heart in a spiritually dead sinner. This truly is the miracle of all miracles, when God takes a dead soul and gives it eternal life! "Praise ye the Lord!"
Thirdly, we are to praise Him as long as He gives us breath. Some think that praise takes place only during a worship service. As we read the Psalms we find David praising the Lord wherever he was at. Sometimes he was in the house of worship and sometimes he was out in the woods fleeing from Saul. Praise the Lord at all times, in all places and in all circumstances. He certainly is worthy of our praise!