As Bill said, shoot a lot, worry about them later. If you end up with 1 usable out of 20 after straightening and cropping, you're doing extremely well for shooting blind. If you've got an add-on wide lens, that'll help to make sure your target remains fully in the frame.
Alternatively, find some place where you can lean against a wall or something, set the focus and exposure for a likely spot, and wait for people to walk through the zone.
Another option, though I've never used it: use the selfie camera to shoot over your own shoulder.