(from Dundalk Md Charivari, Tom Wheatley, editor)
Whole steps are usually spaced too close together when singing the ascending scale and too wide when descending. The same is true for half steps to an even greater degree. Vocalizing on whole tone scales and chromatic scales is very good practice. Vocalizing from half steps up to octaves slowly on "lah" will also improve the problem. Wide intervals and chromatics passages are easy targets for flatting.