I think that when we do certain things as a matter of routine...like putting in contact lenses or setting the alarm, it's easy to forget if we actually did it or if we remember doing it before because we do it on an every-day basis.
I've gotten to where, when I set my alarm before getting into bed, I say out loud..."I set my alarm." Not sure if it helps, but I haven't forgotten to set my alarm in quite some time.