Optional steps :

• Remove unwanted emulators : https://www.reddit.com/r/RetroPie/comments/2wskvq/hiding_emulators_in_emulation_station/
• Configure game save / restore function : https://www.reddit.com/r/RetroPie/comments/2jg2xj/saving_games_in_retropie/
• Add a low battery LED indicator : http://www.stuffaboutcode.com/2016/01/pocket-pigrrl-battery-monitor.html
• In GBA emulator, to replace left / right triggers with X / Y keys, edit /opt/retropie/configs/gba/retroarch.cfg :

  input_remapping_directory = /opt/retropie/configs/gba/
  
  input_player1_y = nul
  input_player1_x = nul
  input_player1_l = x
  input_player1_r = y
  
  #include "/opt/retropie/configs/all/retroarch.cfg"
  

• Edit your /opt/retropie/configs/all/retroarch.cfg to load / save games by pressing select + A or B :

  # Reset the content.
  # input_reset = alt # comment that line
  
  # Saves state.
  input_save_state = ctrl
  # Loads state.
  input_load_state = alt