1) Print the following portrait image and cut it out:
2) When Yuffie joins your party, be sure to name her "Aeris". If Yuffie is already in your party and you didn't name her "Aeris", then you cannot revive her.
3) After Aeris dies, whenever Yuffie, now named "Aeris", is on-screen, put your Aeris potrait cut-out over her face and presto!, you've revived Aeris.
Originally Posted by Flying Mullet
In Final Fantasy VI, after you get the secret books in Cyan's cave in Mt. Zozo if you press the confirm button on the books 255 times each you will see a password in the description box of the items screen where it shows the descriptions for the books, "a book of tools..." or whatever. Write this password down and then go fire up FFVII. Get into battle with a character with a mastered "Revive" materia. Get into battle and kill and revive the enemies in it using Life2 (weaker enemies are easier to do this with), it doesn't matter which enemies or if it's the same one, for a combined total of 255 times (because of the large MP consumption needed, you'll need a lot of Elixirs or such, and the W-Item item duplication trick helps with this). After the battle you will find the Mythril Phoenix, which you take to the guy that collects Mythril. Give him the Mythirl Phoenix and he will ask you if you would like to use the Phoenix's power to see an old friend. Answer "Yes, I would like that" and he will ask you for the password. Enter it correctly and he will tell you that your friend is looking for you. Go to the forests by Junon where Yuffie first meets your party and you will meet Aeris in battle much the same way, except she rejoins you, similar to the way Gau does when he joins your party in FFVI.
(SPOILER)I wrote these in the past but I don't know if they'll count. I'm still proud of them and I had to share them.