When I make a mixtape track, it always starts with the beat... I never wanna use something that I dont feel I can flip harder... i.e. Im not gonna rap on a Mac Dre instrumental to Cal Bear an expect everyone to love it, or California Love by Pac... Also I dont like hearing beats on mixtapes that are hit singles already... Cuz it jus sounds cheesy to re-do em most of the time. I go for the beats that make people say 'uhh! who made this?!?'... An then I follow up with some dope lyrics to show em what I got.
Like someone else said, my favorite mixtapes are the 1's by The Team... even when the beat is over an they're jus talking its still holdin my attention.