I think it depends. Speaking as the most anti-belt person here (for myself, not for others), I can tell you that I do own a belted trench as well as a belted jacket (that you may have seen on the YLF convention blog post). But I have tried other belted coats and jackets that look awful. Here are some of the things I look for:
-Shoulder structure or epaulets. This won't work if you're an inverted triangle or already have strong shoulders, but mine are very narrow and sloped, and having the extra structure or detail in the shoulders makes them look stronger, thereby making my waist look smaller by comparison.
-A sturdy belt. Mine both have wide, structured belts...not wimpy flimsy ones. I also like belts that buckle, as opposed to tie, because if you manage to get down to one of the last holes it makes you look tiny, heh. It's not required though. My coat is a tie coat and the jacket has a buckle. If you tie, it's nice to have lots of the belt hanging off at the end because it looks like you're just so small that the belt is too long for you. But maybe I just read more into this than I should
-A bit of flare underneath the belt. Mine doesn't flare out too much, but it will give that nice crinoline effect and make the waist look smaller. I adore trenches with big full skirts.
-No double breasted styles.
-You could also go for an unbelted trench although they are harder to find. In general, I prefer seaming and structure to obvious details like belts and banded waists. If you find one like that you won't have to worry about belts.
In short, if you find ways to bulk up a little just above and below the waist, the waist will look smaller.
But I have also seen you wear belted outfits, so I'm not even sure that you need to think about this too much. If you can wear a belt on an outfit, I'm pretty certain you can wear it in a jacket or coat.
As a final thought, I always thought men look strange in belted trenches. Talk about drawing attention to an undefined waist! I just think it's weird...
