I keep a spray bottle handy. It has a very fine mist (it was used to spray anti static spray on clothing before I appropriated it for my watercolour work) and I use it to keep my painting wet. If you spray while the paper still has a shine on it you will not disturb the paint and it will allow you to keep working longer. It takes a little practice and observation to get this right, so I suggest you practice using the spray bottle in your home or studio before you use it outdoors.
Mix lots of paint in a decent sized Palette
I have a nice metal folding palette that has large mixing areas when it is open. I make sure I mix more than enough paint to cover an area without me having to mix any more. This gives me a lot more time to work on my painting. I will write more on my watercolor painting kit in another article coming soon.
Watercolor Brush Size
I use the biggest brush size suitable for the area I am painting (see my article on the watercolour progression chart for more information about brush size). This lets me paint very quickly which further reduces the chances of an area drying before I want it to.
I also use round brushes with a very good point. These let me continue to use larger brushes for longer i.e. because my brushes have a nice point on them I can use the same brush for large broad areas as well as smaller detailed ones. This means I spend less time having to change brushes while painting a particular passage.
For you first attempts work on paper which is smaller than you normally use when painting the studio. It will not take too long for you to be able to increase painting size, if you like, but at the start, smaller will be easier.