sounds kind of sad
but i just practise by standing in front of a mirror and trying to keep tight against the edge
Its all about conditioning your off hand
so carry your marker with you off hand etc. when your going to the field
Its also about will power, It WILL be tempting to swap in a game but if you resist then it will a continual improvement.
So my tips
practise in a mirror
practise firing the marker at the TV with your off hand
Practise snap shooting
Practise....
Practise..
practise. get the point
i'm no master but i've been practising for around 4 months now and it is almost as good as my right hand (i'm right handed)