There is a group of irregular verbs conjugated similarly. They are:
dormir, sortir, partir, servir, and most other verbs ending in -mir, -tir, or -vir
Here is a rhyme pattern:
“Drop 3, add -s -s -t” (for the singular forms), “Drop 2, for ils/elles/nous/vous” (for the plural forms)
DORMIR – to sleep
je dors nous dormons
tu dors vous dormez
il dort elles dorment
–Exercise to practice sortir, dormir, partir, servir, and other irregular -ir verbs
-about.com has a table showing partir, dormir, and sortir conjugations
-français interactif has an exercise specifically on partir, dormir, and sortir
It is best to learn these verbs together. Memorize the pattern for conjugation.