Came here to say "tailoring" but these wise ladies already beat me to it. A lot of my tips come straight from reruns of "what not to wear", so my tips are not original LOL. Belts, scarves, statement necklaces are good ways to draw the eye and change the perception of an outfit, so funky accessories have more benefits than just making us smile. Jackets can have the same effect as altering a button-up shirt to make it less boxy; it creates back-to-back C's or an hourglass shape. Stacy and Clinton and their advice helped me make peace with my weight gain when I got sober; thankfully, a lot of their tips are easy and inexpensive to implement.
My last piece of "advice" is this: we humans are our own worst critics. When I think I look funny, I have to try and distance myself emotionally, and just look at the outfit objectively (like, try and imagine what I'd think if I saw it on a stranger). I've also decided that most people are probably not noticing if my shirt is weird or funny or unflattering, because they're too busy worrying about whether or not I think their pants are weird or funny or unflattering. All that to say--you probably don't look as "dumpy" as you think you do.