I know there are a lot of different opinions on this one. Many people live and die by their outlines, while others can't stand to even hear the word, let alone use one. Many of us fall somewhere in the middle.
How do you feel about outlines? Love them? Hate them?
For most of the books I've written throughout my life, I didn't have an outline. When I was a teen, I think about half of my books ended up stalling somewhere around page 120-150, and I would just abandon them eventually. I was writing "organically" (though I didn't call it that at the time), just off the cuff, to see where the story would lead me... and yeah. Many times where I ended up was stuck. And then quitting and starting something new. "Hello shiny new idea, you look like lots more fun than trying to figure out how the heck to dig my way out of that mess!"
That's fine when you're writing for fun. But when you're writing to get published? Not so much.
For me, having an outline is a major help. My outlines vary from notes jotted down on a bunch of different papers (back of receipts in my purse, envelopes from letters we got in the mail, and now this cute little notebook that sweet Ally gave me almost a year ago that I now carry around with me). I have a black full size notebook that I also write notes in, and that I keep all my "stray" notes in as well. For HIDDEN (the book that I told you about in my "journey to getting an agent" series) and it's sequels, I ended up doing chapter by chapter outlines. With as many details and things going on that I needed to keep track of, that really helped me stay focused. I also wrote much faster because I knew exactly where I was going.
Am I saying you MUST use an outline? No. In fact, the book that got me an agent was one that I wrote completely off the cuff, with no idea of where it was going for about the first quarter of the book. I did finally write down a few notes for where I wanted the ending to go about half-way through, but that's it.
So ultimately, it's up to you. But I recommend you try both ways, especially if one of your manuscripts has got you stuck in a corner, dreaming of that shiny new idea.