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Something that solves this problem (and possibly others) is fretting before attaching the board to the neck.
If you fret the fingerboard off the neck, put 3/8" to 1/2" blocks under each end and then clamp the center
down in the middle, leave it overnight, you'll have a perfectly flat board in the morning.
Flat fingerboard+flat neck attached with non-water based glue= only minor adjustment later in the build.
IIRC I stole that idea from Mario Proulx.

Maybe this is unnecessary, but before the fretboard is attached to the neck, I suggest making as sure as is possible that the neck angles are good, both in elevation and longitudinal alignment.

phavriluk wrote:Maybe this is unnecessary, but before the fretboard is attached to the neck, I suggest making as sure as is possible that the neck angles are good, both in elevation and longitudinal alignment.

Seems logical that this straightening is generally unnecessary. If frets are laid in after gluing the neck on, you will still get the same compression tension as seen with laying frets in that cause the bowing. The forces should be the same either way.

Forcing straightness before gluing could possibly also have an adverse impact on the cellular structure of the fingerboard wood along the fret slot kerf. What phavriluk says on geometry, IMHO, is far more impt.