A technique I was told was to squeeze the head of the penis with fingers and bend it slightly (either you can do this or she can). It causes the penis to go a little flaccid and delays any orgasm for a short while but you have to get it in the right place and that is hard to describe. However, you do need to be expert at knowing when you are due to cum and pulling out in order to perform the operation.

There are hollow dildoes and you may also want to consider two sided penis gags - you wear one penis side as a gag and she uses the other one (which is usually longer) to ride your face.