So, to the entry...
So I'm overweight and want to look reasonably good in a suit. Yes I know I should get fit rather than blame the clothes, but that's got to happen gradually - I hear about too many men my age (45) taking to exercise and changing their diets radically and keeling over suddenly. I've lost a stone in the last year and that's steady enough thanks.
Clothing can be a bit of a minefield and I've only really been happy with it since going the tailored route (and losing that first stone). I have what they call "squareback" which is where my shoulders jut out more or less at right angles, too. It's not an off the peg shape. Even with the help of bespoke tailors I have still managed a few terrible sartorial mistakes, though. I bought my first fitted suit one Summer. Linen. So far so good. Light, oatmeal linen.
Oh dear.

No matter what anyone tells you, if you have a spreading midriff then light trousers won't do you any favours. The jacket still looks great - it's a perfect, tailored fit that you can wear as a separate - but I should have stuck with the darker legwear (and bumwear, which is what we're all thinking).
Stripes can also be awkward. I have a pinstripe in a silk/wool mix. It looks good but I chose it because everybody thinks vertical stripes are a bit slimming. Let's leave aside that tests have proven that this simply doesn't work; one glance at a fat bloke in a pinstripe and everyone knows the trick you're trying to pull. It's still one of my nicest suits for presenting, but the plain grey one I bought a little later is better. Maybe because it's not trying so hard.
This is why I've actually concluded suits that work best on a big bloke are the ones in a solid colour, like the blue linen jobbie in the picture, which is half-lined to reflect the summer weather which you might remember. The patch pockets you probably can't see on the jacket give it an air of informality so it works as a sports jacket too when the weather's right - other options are to have flapped pockets but please, please slant them so they don't look like a contour line around your gut.
With a solid dark colour, careful cut (slim fit - this means it's fitted around you rather than expecting you to be slim, the bigger stuff looks like a tent) and slanted pockets I actually end up looking as good as I'm likely to, short of getting back into the gym. At that point I'll need a bit of suit remodelling and look even better. A good style advisor who's honest without being rude while you're choosing is of course invaluable throughout the process.
All of which leaves only one thing to address - the habit Certain Tailors have of leaving a sheet with your detailed measurements in the pocket, with legends like "seat: protruding" and the same for "stomach" - it reads less like a spec sheet, more like a review of your shortcomings. And yes, A Suit That Fits, I'm looking at you.. ;) !