Day One, the excellent, full-featured journaling app for iPad and iPhone (and one of my favorite apps in general), now supports tags and search. Tags are a pretty great way to add some organization to your posts, and there is even a way to turn all hold #hashtags you added into officially supported, indexable tags.
You can also now search your posts using a bunch of handy operators, like the typical “and/or” options, quotes to match an exact phrase, diacritics, and more. Go to Settings > Search > Search Tips to learn what’s at your disposal.