One mistake to avoid when packing your art supplies for travel painting, is to not take too many art supplies. Even though I thought I had pared down my supplies for this trip, I took a lot of things I didn't use.
The only brushes I used were my travel watercolor brushes in 4 sizes made by cheapjoes.com
I discovered that using a mechanical pencil with a good eraser and my Schminke pocket paint set with its own little water container really made a difference when I sat down to paint in an outdoor location. The mechanical pencil was great because it was always sharp and carried its own eraser so I didn't waste time searching for my eraser or pencil sharpener. The pocket paint set was great because it had enough room for plenty of paint colors, and I didn't have to use a separate water container when painting in restaurants or on the airplane plus it was small enough to fit into my supply bag easily.
These travel supply reductions really helped because weight and ease of use are big factors in choosing supplies that you can grab at a moment's notice when you catch sight of something you want to paint while touring around. The weight of your supplies is important too because its very tiresome to carry around pounds of supplies when you are out for the day plus too many supplies take up too much room in your 22" suitcase.