If you long-press on an App Extension in the Action sheet, you can drag it left and right to place it where you want. This is great for reordering one or two extensions at a time.
If you want to reorder many at once, you can swipe all the way to the right and tap More to get a full, draggable list of all installed extensions.
Note that a persistent bug in iOS 8 might eventually reset your custom extension order (hopefully Apple fixes this soon). But in general, your order should stick between all apps.
[thanks Tyler Mumford!]