Disguise letters and words by painting them in a virtually identical color as the background. In this way, you camouflage the text against the surface on which it's painted.
Represent letters as "real" objects within the subject of the painting. For example, you could paint a giant letter "A" as an abstracted entranceway, or an "M" as a mountain ridge. In this way, the letter assumes two forms: itself and the object it masquerades as.
Place the letters at the periphery of a painting, away from the viewer's attention. Pablo Picasso experimented with this technique in his famous 1911 painting "Ma Jolie," in which the words "Ma Jolie" are stenciled at the bottom in dark lettering, away from the focal points of the painting.
Render the letters in a very small size, observable only to the most scrutinizing eye. This allows the "secret" placement of letters in art while rendering them essentially invisible.