1Praise ye the LORD. Sing unto the LORD a new song, and his praise in the congregation of saints.
2Let Israel rejoice in him that made him: let the children of Zion be joyful in their King.
3Let them praise his name in the dance: let them sing praises unto him with the timbrel and harp.
4For the LORD taketh pleasure in his people: he will beautify the meek with salvation.
5Let the saints be joyful in glory: let them sing aloud upon their beds.
6Let the high praises of God be in their mouth, and a two-edged sword in their hand;
7To execute vengeance upon the heathen, and punishments upon the people;
8To bind their kings with chains, and their nobles with fetters of iron;
9To execute upon them the judgment written: this honour have all his saints. Praise ye the LORD.
For clarity, here's Matthew Henry's insight on the first few verses: "New mercies continually demand new songs of praise, upon earth and in heaven. And the children of Zion have not only to bless the God who made them, but to rejoice in him, as having created them in Christ Jesus unto good works, and formed them saints as well as men." I don't have a lot to say about that, but I know that in the past, I have praised and continue to praise Him for all the good things He has done in my life -- such as freeing me from my darkest moment and helping me out in my math class. I sung a new song to Him by praising Him for helping me out, which would also count as rejoicing and being joyful in Him (v. 2). I don't really use instruments to praise the Lord (v. 3), but I know that when I'm church on a Sunday, there are some that do as part of the worship. I guess, though, that that verse reminds me just to praise God just for being God and being good, which I can do with or without others. Since God takes pleasure in me (v. 4), I give back to Him by rejoicing in Him. I believe that some of the things I do to illustrate that I rejoice (or take joy in Him) include following His plan for my life, reading and obeying His Word, and praising Him. I don't really have to go into detail about those, but I'm glad that I can praise and worship Him in more than one way. I'm not sure what I'd say about verse 5 except that I rejoice in the glory (or honor) of the fact that He takes pleasure in me. For clarity, here's Matthew Henry's insight on verses 6-9: "Some of God's servants of old were appointed to execute vengeance according to his word. They did not do it from personal revenge or earthly politics, but in obedience to God's command. And the honour intended for all the saints of God, consists in their triumphs over the enemies of their salvation." So basically I'm not gonna kill someone for vengeance reasons and say that God told me to do it. However, I did face a spiritual during my darkest moment and trust in God's Word was what helped me to triumph. In terms of verse 6, I am reminded of Hebrews 4:12, which says, "For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart." Taking both verses into account, I just continued to trust in God during my darkest moment -- which I see as form as praise to Him since I wasn't being arrogant by trusting in myself. Trusting in Him was what helped to puch through my drakest moment since I knew He was gonna eventually deliver me. Verse 9 is interesting because by trusting in God, I knew that He would take care of the enemy and have vegenace upon him (V. 7) and He did. That's what the verse means to me anyway. And eventually, when the time for the enemy's demise comes, I wouldn't be surprised if part of the reason of includes the fact that enemy tried to influence and mislead God's people. The part that says "the judgement written" is an allusion to Numbers 24:17-24, which I'll include for the sake of context...
17I shall see him, but not now: I shall behold him, but not nigh: there shall come a Star out of Jacob, and a Sceptre shall rise out of Israel, and shall smite the corners of Moab, and destroy all the children of Sheth.
18And Edom shall be a possession, Seir also shall be a possession for his enemies; and Israel shall do valiantly.
19Out of Jacob shall come he that shall have dominion, and shall destroy him that remaineth of the city.
20And when he looked on Amalek, he took up his parable, and said, Amalek was the first of the nations; but his latter end shall be that he perish for ever.
21And he looked on the Kenites, and took up his parable, and said, Strong is thy dwellingplace, and thou puttest thy nest in a rock.
22Nevertheless the Kenite shall be wasted, until Asshur shall carry thee away captive.
23And he took up his parable, and said, Alas, who shall live when God doeth this!
24And ships shall come from the coast of Chittim, and shall afflict Asshur, and shall afflict Eber, and he also shall perish for ever.
I'm gonna let the passage speak for itself, but I thought the parallel was interesting. I figure God has a judgement in mind for those who opporess His people and will carry out the judgment(s) as He sees fit.
Henry, Matthew. "Concise Commentary on Psalms 149". "Matthew Henry Concise Commentary
on the Whole Bible". <http://www.searchgodsword.org/com/mhc-con/view.cgi?book=ps&chapter=149>.