Write down everything you know about the poem, including the poet's name, the title, phrases, entire lines, and unique words or phrases contained in the poem.
Perform Internet searches based on the poet's name. There are many websites that have a collection of poems by various poets. Then, search through the poems for words or phrases you recall.
Search by using unique words and phrases. For example, if you search for "fog comes" and "cat feet," you will locate Carl Sandburg's poem containing the line, "The fog comes / on little cat feet."
Use completed lines to contextualize poem. For example, by including multiple complete lines in a search you will receive the conjoining lines in your search results. Most likely, with these results you will be able to find the full poem and title.
Search through collections, anthologies, and indexes at your local library. Often there is nothing more than you can do than read poems to try to identify the verse you recall. A poetry collection usually has between 30 and 60 poems, and not all reach the internet. Try asking your local librarian; perhaps she is familiar with the poem.
Request a search from a poetry forum. There are many websites where you can talk with qualified poets and professors whose extensive knowledge can help you identify the poem.