Rain? What's that?
Leaves, lots of them, any kind.
My recent experience indicates they are much better water retaining and soil temerature moderating mulch than shredded hardwood mulch.
Don't worry about myths that certain leaves/pine straw are bad for your soil or that they will never break down, not true. Nature hasn't heard about those myths. Must have been made up by people who want to sell you their products.
They may not be broken down by the fall when you want to plant fine seeds, but easy enough to push them out of the way. They are free and lightweight!
Also scattering good sized branch trimmings around your beds will also really help reduce evaporation. Living plants also provide cover.
Bare earth is the worst.
Any other helpful suggestions?