First, make a plan of what you are going to eat. It’s quite tempting to just eat on-the-fly, so to speak, and just graze. But, this can be a real problem if there’s lot of food left out and you’re at the picnic for hours.
So, plan ahead. Something like:
You’ll have two hamburgers, water and baked beans. (Mmmm!) Skip the sodas, potato salad and desserts.
Perhaps, if you get hungry later, some of that salad. (Green salad, not potato or egg, okay?)
Drink LOTS of water as you’ll need to stay hydrated and it’ll keep you full.
So, have fun at that picnic and save a burger for me, okay? (Medium, please.)