My ex-wife and I discovered by accident. What started out as some strenous fingering merely slipped into a fisting event. She loved it! After that it was something she insisted on, so we had to figure out how it make it happen easily. This is what I found to work best. Sorry if it's a little clinical.

Several of you have already mentioned the need to build to it, this is so very true. Start with on finger, build slowly adding one at a time until you reach her limit. You may only get to two or three on the first time. The trick is to expand her 'vertical' capacity to take in your fingers. By vertical capacity I mean that you should insert your fingers vertically -knuckle line parallel to her hips/vaginal folds- not side by side. Despite what most fisting shots allude to, no one is going to ever call 'hotizontal' insertion knuckles parrallel to her hips- sensual or even comfortable.

At some point, if her hip structure will allow it to progress, you should reach a point where you have all four fingers inside her in such a manner that you can place your thumb squarely on her clit. Once you get to this point, you're 3/4 of the way there.

To proceed, cup your handslightly and fold your thumb across your palm. This will reduce the profile of your hand and you should be able to slip the whole thing inside her.

Once you're in, the fun can really begin. By balling your hand into a fist -thus the term- you can touch several parts of her she never knew she had all at once. The orgasmic results are intense. Another aspect to consider is the stroke. By working your fist back and forth you can litterally fist fuck her. My ex claimed to love the feeling of almost having my fist pulled out of her and then shoved back in. Be careful, however, as the out stroke can pinch very sensitive tissue between your fist and her pelvis. If you hurt her, she will never ask you, or let you, do it again.