What's the difference between "guardar" and "mantener"? The English I want to translate is "Help us to keep a good sense of humor." The verb to translate is "keep".
Guardar is keeping something safe, like keeping money in your locker. This would be used in the same context as "store".
Mantener is keeping something the way it is, like keeping your locker clean. This would be used in the same context as "maintain".
In this context, "keep" is used like "maintain", so a good translation would be:
Ayúdanos a mantener un buen sentido del humor.
Another translation I would suggest uses the verb "Conservar", according to its third definition in RAE's dictionary:
3.tr. Continuar la práctica de costumbres, virtudes y cosas semejantes.
So, my suggestion is as follows:
Ayúdanos a conservar un buen sentido del humor.
Closely related to the context of the sentence we are dealing with here, for example when you are already talking about keeping that good sense of humor in former paragraphs, then you should use de definite article, like this:
Ayúdanos a conservar el buen sentido del humor.