But when sensei has us do a technique thats extremely complicated, difficult, or very new to me, I try to find someone with a hakama or a brown belt, or who I know is more experienced than me so that I don't spend the entire time we have to work on the technique trying to figure out the first half of it.
good point, traditionally it should be Kohai's responsibility to seek out sempai to train with, it is quite often the "easiest" way to improvement, and it shows initiative. BUT Sempai, should not show any attitude that they have to work with Kohai, after all they were there once, and that is Sempai's responsibility,
During regular training, I try to just work with whoever is sitting next to me. If there's something I didn't "get" during the class, I'll work with someone, perhaps the teacher, after class.
but more often than not, I too just grab whoever is next to me.
I think it all works out in the long run...