Tom is on the right track, but you don't want an indicative form, which 'es' is. His Latin sentence says "you are yourself and humble."
An imperative would work, but the form for the 2nd person imperative of the verb 'to be' is rather rare, and creates some other problems not worth getting into.
If you want to be quite literal, a better solution would be to use a jussive subjunctive:
Sis ipse et humilis.--"May you be yourself and humble."
While grammatically correct, this does not strike me as how a Roman writer would have expressed this sentiment.
The options below are perhaps better:
Esse te ipsum verum atque humilem est optimum.
To be [your] true self and humble is best.
Conare esse te verum atque humilem.
Endeavor to be your true self and humble.
While Tom is right that humilis can have a negative connotation, I think it is closer to the intended meaning here than modestus, which means something more along the lines of reserved.