The schwa is a neutral sound, (no distinctive characteristics), but it is the most common sound in the English language. To make the uh sound, put your hand on your diaphragm and push until a grunt escapes. Don't move your jaw, tongue or lips, just allow the sound to flow past your vocal cords. It should sound like uh, not ah.

Once you master the two sounds [æ] and uh, you will have an easier time pronouncing 'can' and 'can't'. In a sentence, the simple positive 'can' sound like [k'n]. The simple negative 'can't' sounds like [kæn(t)].