Well yeah, you do have to perform a trade, but you don't have to trade your beloved Level 100 Shiny Porygon for it to evolve! aljames managed to compose a nice guide, so I give complete credit to him. Here it is:
1. If it needs an evolution item, equip the item before you even start. If the Pokemon doesn't use an evolutionary item to evolve, go right ahead to Step Two.
2. Deposit it at the GTS, and ask for an impossible-to-obtain-Pokemon.
3. Go back to the menu where you see DEPOSIT POKEMON, SEEK POKEMON and EXIT.
4. Choose SEEK POKEMON. Seek a Pokemon (such as Wurmple) and perform a trade.
5. After the trade, go back to the menu where you see SUMMARY, SEEK POKEMON and EXIT.
6. Choose SUMMARY and take back your Pokemon. It will evolve now if you did this correctly..
You need a Wi-Fi Connection, and that's about it... I hope that this helps to keep Wi-Fi Scammers at bay, and to keep the Blacklist Thread in the Battling/Trading Section as slim as possible!