1. Talk to them as if they can understand. Even if they can't at first -- they'll come along in time. Chummy was so canny we weren't even sure of ourselves spelling words out in letters, at times. (C - A - R)
2. Don't expect retrieval: the Boxer job is to find the frisbee/ball/bunny toy and stand over it so it doesn't get away. Retrieval is YOUR job.
3. Walks must be interesting in their own right or forget it. I have seen neighbours walk their dogs with a sense of duty and boredom round the block and a Boxer will simply not do that. Their attitude is: 'Show me the squirrel/rabbit/tortoise/waterfall/lake with birds in it or else don't bother'. Can't blame them, really.
4. You never have to insist. If you have to insist, grumpily, you aren't doing it right.
5. Play is the key to everything you want to instill.
6. Training is what you do to raise the Boxer: once raised, you can both nearly forget about it. A Boxer dog really does grow up in the sense that you can dispense with most commands, most of the time -- we did.