I just try to get one randomly wrong. It almost always lets me through. Also, I'll pick the wrong one that almost looks right. Say, a picture with a vaguely traffic light shaped mailbox or a train shaped car or something. :)

I have noticed there's a very very low correlation between how hard I try on captchas and how quickly they let me through. I just quickly mash a bunch of tiles to start, maybe trying to be in the right general area. That works surprisingly often and when it doesn't I, and pay closer attention to what I'm selecting, then I pretty much always get through on that next try.

This approach mostly fixes the annoying phenomenon where I carefully select the exact right tiles only to be told, "too bad, try again".