How should I select a trail running shoe?
Selecting a great trail running shoe is a combination of personal preference and design features specific to what you want to do. Firstly you want to ensure that whatever you choose has a snug fit around the middle of your foot (the arch area) and also holds your heel firmly in place, when you're running over uneven terrain you will quickly tear up your heels if your shoes don't keep you heels from lifting in the shoe, this will also leave you prone to injury like rolled ankles as your feet won't be as protected if they're allowed to move around.
Stack height is one of those preference things, typically a higher stack (this is the thickness of the sole material) provides more cushioning and protection from uneven and debris laden trails, and a lower stack gives less protection for your feet but gives you a more natural feel, there is also much debate over improved biomechanics from this design as well. Try a selection and get a feel for each one.
Whether you opt for an aggressive, deep tread, or a more low profile design with smaller/shorter cleats (or lugs), this will largely be determined by the trails you intend on running, and you may want to have more than one pair to choose from to cover all bases. If you're keen on the more wet and muddy trails, obviously a deep lugged tread design to cope with this is your best option, but if you plan on sticking to more flat, dry trails then a more subdued tread design will be fine.
Drop is another area of preference, you can get zero drop trail shoes so your heel is at the same height as the ball of your foot, and you can also get trail running shoes with a higher heel, up to 12mm or more above the toe height. You may need to try a few before you decide on what suits you best.
Regardless of some of these factors mentioned above, fit is by far the most important one, the rule of thumb, other than just being comfortable, is to allow a space about the width of your thumb in front of your toes, remembering it should also be snug around the middle of your foot and your heel held firmly.
It's always recommended to try on several pairs before you buy, and the longer you can have them on your feet before making your choice the better. It's important not to rush your decision as poorly chosen footwear can be unforgiving once you've put them to work, so take your time! Once you find a pair that suits your needs it can also be a good idea to purchase a second pair so you can rotate them, this takes the pressure off trying to dry them in a hurry, etc. and also means you'll get twice as long out of a trail shoe that you love - it may not be there next season!