how do you memorize a piece of music?
i have a heard time remembering solos especially
Practice, Practice, Practice.....if you play it through enough you WILL remember it. Repetition is the basis of memorization. with a hard song that i have trouble with i will sit down and play note for note very slowly and gradually speed it up once i start to get it.

eat fish - they say its good for the memory

if the song is a bunch of single notes then it can help to pick out the chord shapes. this way you can remeber the progression and the flow should come naturally if you know the song well enough.
lol. well, then Niacin supplements. Im pretty sure thats the memory goodness in fish. Practice is probably your best bet though.