This one is from my friend George Graham over at AcadianaTable.com. It's in an essay he's written for the November/December edition of the magazine, but it's too good to keep.
When you need to cut almost, but not quite, all the way through a dense vegetable, piece of fruit, loaf of bread, etc. pull out a pair of chopsticks. Place them on either side of the portion to be sliced; they'll make a perfect guide, stopping your knife before you've sliced all the way through.