Well this aint really a mnemonic but it helps you remember the cranial nerves:
God gave us 1 nose (olfactory) and 2 eyes to see with (optic). 3,4,and 6 make your eyes do tricks (oculomotor, trochlear, abducens). 5th cranial nerve is recalled by remembering 5 rhymes with tri (trigeminal). The 7th nerve can be visualized by a 7 (top on the forehead and the bottom going across your face). Think of number 8 fitting into your ear (vestibulocochlear). When you evaluate nerves 9 and 10 under your chin (glossopharyngeal, vagus) note that both these resemble the letter g and that checks into gag reflex. For 11 (accessory) visualize a 1 on both shoulders that should remain in place when you shrug your shoulders. Finally for 12 (hypoglossal) visualize a person sticking their tongue out from side to side saying the end.
Again not really a mnemonic but it helped me remember the cranial nerves. Also I should note that the nerves are lettered in roman numerals (I,II,III,IV,V etc.).