The first is by Antione de Saint Exupéry, author of The Little Prince and a pioneer in aviation:

Perfection is not when there is no more to add, but no more to take away. 

The second is by William of Occam (1288-1347)  You might know the name from Occam's Razor.

It is vain to do more with what can be done with less.

These quotes mean a lot to me and were found, most recently, at the beginning of Chapter 5 of Tim Ferriss' book The 4-Hour Workweek.  More on that later.