Here is what you have to do to bring Aeris back to life. Then enter the Aeris resurection code 8009B376-FF03 then start the game with the code on. Now when the title screen appears turn off the Gameshark. **Dont use this on the first CD, it might freeze up/glitch the game.** Then load a saved game on either the 2nd or 3rd CD. Next go to PHS to remove the 3rd character on the party screen. Now turn the Gameshark back on and Aeris will appear! (you should probably turn the Gameshark off and save your game so you will always have Aeris) Be careful though, the man in the air ship after you return from the mako crator will remove Aeris so you will have to do the code over if this happens, but if you are past this scenario you shouldn't have any problems.
The code is 8009B376-FF0A. This is the code to use Sephiroth. First start the game with the code on.Then when the title screen appears turn the Gameshark off. Next load a saved game on the 1st or 2nd CD. Next go to PHS and remove the 3rd character on the party screen. Now turn the Gameshark back on and Sephiroth will appear! (You probably want to turn the Gameshark off and save your game so you'll always have Sephiroth)
Once it is in the first slot, equip it on all three characters, so you'll have three of them. and then turn the code off. Since it will be inserted into the first slot, you should only enter one of these codes at a time. If you are having trouble with the game these codes sure will come in handy!