I am not sure what it is about boxers and their feet, but my BF said his other boxers were very sensitive about nail clippings too.
We found that it usually takes two people--one to get her attention with peanut butter and lots of love and kisses while the other clips. We can usually only do a little at a time before she gets upset. BF will hold her in a cuddle position and I get her hind legs, then a few hours later, we will do the same and I will get the front feet. It takes patience and lots of loving!! The worst thing to do is get frustrated because it only stresses the dog out more.
You might want to have the vet show you how to do it the first time. Also--before you try it on your own, you might want to have some styptic powder on hand. You can buy it at any petstore. If you do hit the quick and it bleeds, you put a little of this powder on and it will help stop the bleeding.
Another tip--the longer you let the nails grow, the longer the quick will be and the more likely it will be that you will cut the quick. Best practice is trim them on a regular basis--taking little bits off at a time.
You can do a search for clipping nails here and there are a lot of great threads with tips and techniques that might help you! Above all, make sure you and your baby are comfortable with it. If you aren't, it might be worth the $10-$15 your vet would charge!

Good luck!