To get 3.3V power from 5V, use an LM1117-N-3.3, similar to what the Arduino does (or the same LP2985-33). You waste no more than 10 mA, unlike using a Zener. Need two 0.1µF bypass caps.
OK, I have had (2025) to correct my earlier advice. Resistors are just fine for higher speed but you must use sufficiently "stiff" resistors, - relatively low values.
For dropping 5 V to 3.3 V logic, you can use a diode and a resistor rather than two resistors. This also overcomes all problems about the 3.3 V side being unpowered. Given that the Pro Mini is capable of driving (sinking) 20 mA, the resistor could be as low as 270 Ohms but for most purposes, 1k should give sufficiently fast response, Diode would be a 1N914 or so, pulling down the 3.3 V input to 0,7 V should suffice.
The bidirectional level shifter I previously suggested is not appropriate in this case, but is very useful for I²C level conversion.
You can buy a couple on eBay or Aliexpress. Pretty cheap.