You have to sell or mail everything to your progenitor from your bank and bag's Item and Crafting section, including equipped gears on the reincarnation. Discard quest items and discard quests. You have to remove (don't sell, don't discard) runes from the toon (Click the runes with Right mouse button and they'll go in Boutique tab of your bag and you can equip them back when the new reincarnation is ready).
Also as mentioned before, make sure you have either returned or taken back shared mount, if you have a best friend. Take off trinket - if it's cash shop one, it'll be safe, if it's quest given, you have to sell it. Remove specks of light, dragon tears etc.