I use wingnuts with outboards, and this is the stupid trick I use...
I usually keep the cable about a 1/4 or 1/8 turn counterclockwise of where I want them to be, then when I tighten the wingnut as far as I can by hand, I also give it that extra 1/4 or 1/8 turn with the cable and wingnut at the same time, giving me a little more leverage, and clamping it down nicely. I check them often, but it seems to work for me. I would think a little nudge with some pliers would do the same thing though. as mentioned, I also had a post melt on my houseboat once, and the battery was a nice 27 group (like a $100 battery or so) that was about a week old and was instantly trashed because I didn't make sure it was tight. oops...