unsigned addr;
unsigned char low,high;

addr = 0x1234;
low = addr & 0xff; /* or low = (unsigned char)addr */
high = (addr >> 8) & 0xff; /* or high = (unsigned char)(addr >> 8) */

The masking with 0xff is more compatible with architectures with funnier word and character sizes, and a good compiler should note that masking with 0xff is a null-operation when assigning to an 8-bit variable.