Psalm 9 should be named the Psalm of Remembering and Forgetting.

Try to notice when you read Psalms, but any Scripture really, a repetition of words or ideas. The Hebrew word for Remember or Memory is Zeker H2143 – zēḵer, or the root word Zakar H2142 – zāḵar. The Hebrew word for Forget is Sakeh H7913 – šāḵēaḥ, or the root word Sakah H7911 – šāḵaḥ. Notice how in Hebrew they sound similar, thus making a rhyme poet like effect. 
Zeker – Zakar – Sakeh – Sakah.

This is where the words are used in Psalm 9, replace remember/memory with Zakar and forget with Sakar and see how it sounds:

:6 The enemy is no more – ruins everlasting! And You have uprooted the cities; Even their memorial H2143 has perished with them.
:12 The seekers of bloodshed, he remembereth H2142 them: he forgetteth H7911 not the cry of the humble.
:17 The wicked shall stay in Sheol, and all the nations that forget H7913 Elohim.
:18 For the needy shall H7911 not always be forgotten: H7911 the expectation of the poor shall not perish for ever.

Actually we have explained the word Zakar before in Genesis 40, that REMEMBER and MENTION are the same word in Hebrew -ZAKAR (click here to read more). To remember, think, bring to remembrance, record, mention. A great verse is in Exodus 3:15, “this is my name for ever, and this is my memorial (H2143 – zēḵer) unto all generations“

Malachi 4:4 “Remember (Think and Speak) ye H2142 the law of Moses my servant, which I commanded unto him in Horeb for all Israel, with the statutes and judgments.”

Here is another example of Hebrew rhyme & poetry with Sakah:

Hosea 8:14 For H7911 Israel hath forgotten H7911 his Maker
Hebrew says : šāḵaḥ yiśrā’ēl šāḵaḥ ʿāśâ (Forgotten! Israel has forgotten his Maker!)