Be prepared! There's nothing worse than heading outdoors to paint to find out after you've set up ur easel that u forgot ur turpentine, worse, brushes!
Make a list and stick to it, take ur time when packing ur stuff, make sure u have everything. Pack the day before so u don't have to rush, even better, have a package ready to go at a moments notice. To be that organized means u have to unpack after ur last outing and prepare for the next. I have a cart on wheels, so convenient, my french easel fits in there perfectly with a fold up stool. I also use bungee cords to attach more paraphernalia.
My list... french easel, canvas, oils, brushes, stool, turpentine, paper towel, disposable palette, cling wrap, (to cover your palette when u leave), umbrella, sugical gloves (keeps the lead at bay), sunscreen, bug-off spray (a must have!), dress for the weather, don't forget refreshments! oh and one more thing, enjoy!