Some have described this as the longest sustained imprecatory prayer.

A Psalm of David

109:1-5 He describes his mistreatment at the hands of others
They spoke against him constantly- vs. 2, 3, 4. They lied about him- vs. 2. They spoke words of hatred- vs. 3. He did good to them, but they did evil to him- vs. 5.

109:6-20 He prays that judgment may fall on them.
Notice how many of these judgments he requests tie to the concept of lex talionis, the punishment fitting the crime.
vs. 6 accuser- They acted as his accusers (vs. 4) so may an accuser stand at his right hand.
vs. 7 guilty- This word is translated wicked (vs. 2, 6). Since they are wicked, may they be found to be wicked (guilty).  
vs. 9 prays for his death but he has put others to death (vs. 16). This is what God warned would happen in Exodus 22:22-24; Prov. 22:22-23.
vs. 12 Let none show lovingkindness to him because he did not remember to show it to others (vs. 16).
vs. 17 "He also loved cursing, so it came to him; and he did not delight in blessing, so it was far from him."
The fact we see so many instances of this, lead us to think that the curses called down upon this wicked are the same things he inflicted upon others.

109:21-31 God's intervention changes everything
vs. 21 But You, O God, the LORD
vs. 22 David acknowledges he is afflicted and needy and desperately in need of God's help.
 vs. 27 He prays for God to deliver him in such a way that all know that it is God's work.
vs. 30-31 He promises to praise God, who stands at the right hand of the needy. See right hand in Pss. 109:6 and 110:1.