The best practice (in my opinion) is you get people to cheese the hell out of you and see if your build can hold it if you're sorting out your early game. Lategame is playing against certain styles so it's nice to have someone to grind a style against you without deliberately making changes as they know how you play.
What I can't seem to find is someone who (if I lose to something) will do the same thing again so I can see if I can practice against that exact thing. It's really annoying, but it's what I sometimes need to practice and it's really hard to find people willing to keep doing the same thing over and over until I hold it properly.
What league are you? I love practicing like that.