What Phil said to do is actually similar to what my mum made me do as a kid. If I ever spoke through my nose, she made me hold my nose and speak until I sounded normal. It had nothing to do with foreign languages though - she just finds nasal voices really annoying!

I don't know how to help you but I wonder if you can mimic other English accents, for example, southern England, or Australian?  

I found a similar exercise in a vocal training textbook: count from one to five normally, then when you continue from six to ten, pinch your nose. If you have a nasal tone in your voice, it'll be very obvious. If you don't have a nasal tone, your voice won't change at all. Then try counting from one to ten with your nose still pinched. You might be able to drop your voice into your mouth to avoid the nasal resonance.

Ohh that nose pinching idea sounds great thanks guys.