I think Restof10 meant to say go slow and *DON'T* just thrust like they do in the movies, at least not until you're ready. :-)
I've been anal fisting for over 7 years, been with my current partner 5 1/2 of those, and she has only reached her elbow and felt the inside of my ribcage a half dozen times. At least half of the time she fists me, she doesn't go any deeper than her wrist, the other half she'll get about 4" up her forearm before my body says that's enough. Many times I feel like I want more, but I'm simply too tight, not relaxed enough, or my body simply won't allow it. Here are some hints to help you go deeper if you like.
1) PATIENCE... and *lots* of it. Don't rush it. Play when you can, enjoy when you play, and progress as you're able. It's not a contest and noone is judging you if you don't reach that porn star depth. Just take your time and enjoy whatever you are able to do.
2) Stretching... Play with some fairly large toys as you are able. The first barrier your partner runs into will be your anus. If she can't make it past there, she won't be able to go deeper. I like to insert a 3" diameter toy at least once or twice a week, holding it 10-30 minutes enjoying the stretch on my anus. Your body will try to cramp and push it out, but if you can hold it until the contractions pass and relax, it will strengthen and stretch your anus, allowing you to take larger things (like an arm).
I haven't been able to confirm this in a medical manual, but my partner described a second sphincter about 9" into my bowels. When she reached there, she massaged it first with the tip of one finger, then another, then another, and so on, gradually opening it until she was able to insert her whole hand. This took us months to do, but with patience and practice, it was possible. I like to use a 24" long dildo to reach and open this sphincter and stay accostomed to the feelings.
3) Position... On my back with my knees drawn up is a *terrible* position for me. My bowels and abdominal muscles fall toward my back, folding over themselves. I have taken a deep fist on my stomach, but my favorite position is on my knees and elbows, back straight, chest slightly below my hips. This lets my abdomen stay mostly straight and gives me room to move as needed.
My partner prefers to sit behind and to my left, using her right arm for two reasons. First, the bowels go in a certain distance from the anus, then turn toward your (the recipient's) left side. As my partner reaches this point, she can bend her wrist to keep her hand pointed in the right direction to move my bowels as needed. As pyewacket said, your colon will move, but it takes practice. Second, she's right handed. A deep fisting takes effort from the giver, not only the recipient. My partner's left arm and shoulder are not as strong as her right and tire more quickly. She also has more dexterity with her right hand, which is important for moving with my body, rather than forcing her way in.
4) Partnership... It takes two to fist beyond the wrist, the giver and the recipient. As you've doubtless learned already, you work together, learning each other's bodies, moves, and cues. The more you get to know each other, fisting and otherwise, the easier it will be to work together.
4) Cleanliness... This may not be as much of an issue for you and your partner, but it is for me and mine. If I'm dirty, there is much more friction and discomfort. Also, my partner doesn't like dirt. If there is much dirt, she stops, plain and simple. I'm not excited by the dirt, I just accept it as part of the play, but it is a show stopper for her. As wrestler wrote, "fisting is an intense relationship between two people," and you both have to be comfortable if you want to explore it deeper (pun intended).
Take your time, be patient, and practice, practice, practice and eventually your wife will be able to tickle your belly button - from the inside.