Use a mixture of mineral oil and paraffin wax. The wax is available as a small block in the canning section of the grocery store. Use about half-and-half oil and wax. Place the oil and wax in a used soup can and warm it very carefully (it's flammable!) on the stove. The wax will melt into the oil at a relatively low temperature. Then, wear rubber gloves and rub the mixture onto your chopping block using a rag or paper towel. After the mixture solidifies on the block, scrape it off with a flat spatula and rub down the block to nice smooth finish.
The importance of the wax/oil mixture is that the oil seals the wood and the wax fills in the tiny cracks on the surface of the wood and between the blocks, thereby preventing moisture and bacteria from getting into the block.
I keep the oil/wax mixture in a covered tin can in the fridge. It's easy to reheat and re-apply the mixture.