How to hold a pencil?

We have been trying to encourage our five year old to hold his pencil properly with the correct triangular grip but he prefers to hold it with his middle finger and thumb rather than his index finger.

So what have we tried?

In the Reception class they were told to use "tweeting" fingers to hold their pencil - so "tweet" like a bird with the index and thumb and then pick up the pencil - the novelty wore off! He recently came home with a paper aeroplane and worked out that it was the same fingers he needed to hold the pencil with so that might be another good exercise for children needing to encourage their index finger and thumb to work together.

We have tried triangular pencils but because his grip is only slightly off this doesn't help, ditto the triangular grip. The grippy pencil grip looked promising to start with - it did make him hold the pencil correctly but he found it awkward to position by himself each time he needed to pick up the pencil. Lastly, we tried the tri-go grip this has three clear spaces for the fingers and he could position is fingers independently and most importantly it did improve his grip.

What works best now? Telling him if I see him holding his pen incorrectly I will tell Miss B he needs to have more handwriting practice!

How have you encouraged your children to correct their grip?