Connect a 2k2 resistor from each control signal (RS, E and RW) to the possitive power supply line of your micro (Vcc). The P0 port is unable to output '1s' in its lines without the help of these external resistors (in most 8051 derivatives). In addition, the LCD control signals require fast transitions (at least one of them, i can't remember which one at the moment), so resistors of more than about 2k2 might not work.