I have not yet read anyone else’s answers so this is just what I do.
If it’s a family party, it’s assumed that each potluck guest will take their own un-eaten food home. And the host, will ususally ask if anyone wants any of their leftovers. Everyone knows my Dad cannot resist anything chocolate, so we often end up with more food going home than we brought. I usually bring entrees and salads, but bring home desserts : P
If it’s a work potluck, then I personally usually bring a bunch of disposable plates, take-out containers, foil, and other storage items (some that we keep at work for this purpose) and then load the leftovers into containers (with labels so people can distinguish between vegetarian stuff and everything else so they don’t have to open every single container). And then put them in the fridge. Sometimes I’ll just load up individual dishes, like spaghetti or salad. Other times I’ll make up vegetarian meals and meat meals, because there’s often other people that couldn’t come to the party, because they had to be at he front desk, or guards that come in later. Sometimes I’ll even write people’s names on them, if I know who will be coming in later. Then I’ll leave a big sign on the fridge, announcing which containers are up for grabs.
If the potluck is at someone’s home, who is merely an acquaintence, or if I’m invited by my friend to someone’s home who I don’t know, I’ll bring my dish in my own big tupperware, in my cooler, then I’ll bring along a big zip-lock bag and keep it in my cooler until the party is over. If there’s anything leftover, I’ll spoon my dish into the ziplock bag and put it in the hosts fridge (with their permission) and let them know that they can eat it, or dole it out to any of the guests who would like to take some home. Or if they don’t want it, then I’ll take it home.