How do I shift voltage levels?
Microchip has this document with 19 tips about mixing 3V and 5V parts in a circuit (not just level shifting).
The tips:  


  
  Powering 3.3V Systems From 5V Using an LDO Regulator  
  Low-Cost Alternative Power System Using a Zener Diode  
  Lower Cost Alternative Power System Using 3 Rectifier Diodes  
  Powering 3.3V Systems From 5V Using Switching Regulators  
  3.3V → 5V Direct Connect  
  3.3V → 5V Using a MOSFET Translator  
  3.3V → 5V Using A Diode Offset  
  3.3V → 5V Using A Voltage Comparator  
  5V → 3.3V Direct Connect
  5V → 3.3V With Diode Clamp  
  5V → 3.3V Active Clamp  
  5V → 3.3V Resistor Divider  
  3.3V → 5V Level Translators  
  3.3V → 5V Analog Gain Block  
  3.3V → 5V Analog Offset Block  
  5V → 3.3V Active Analog Attenuator  
  5V → 3V Analog Limiter  
  Driving Bipolar Transistors  
  Driving N-Channel MOSFET Transistors