The suit you can still wear in fifteen years

Buy mid-gray. Plain weave worsted wool, somewhere around 10 to 11 ounces. Two buttons, notch lapel, no more than a light structure in the shoulder. Get the shoulder right on the rack and let a tailor do the rest. If a second pair of trousers is offered, take it.

That's the whole thing compressed. The rest of this is why, and the specific places it goes wrong.

Fifteen years is a realistic target, but only under realistic conditions. A suit worn five days a week is a work uniform and it'll be finished in three or four years no matter what you spend. A suit worn twenty or thirty times a year — weddings, funerals, interviews, church at Christmas, the occasional dinner where you want to look like you made an effort — will outlast the car you drove to buy it in, if you look after it.

The shoulder is the only part you can't fix

Everything else on a suit jacket is adjustable. Sleeves shorten. Sides come in. Length comes up, within reason. The collar can be made to sit flat. But the shoulder seam is built into the architecture of the coat, and moving it means rebuilding the jacket from the inside out. No high street alterations counter is doing that, and no tailor will do it cheaply.

So on the rack, ignore how the waist feels and look at two things. The shoulder seam should end where your shoulder ends, at the corner, not hanging off the side and not cutting in above the bone. And the collar should sit against your shirt collar with no gap when you stand naturally.

If those two are right, buy it and let the tailor earn their money. If they're wrong, put it back, even if it's the last one in your size and it's marked down heavily.

The chest matters too, but differently. Button the jacket. You want to fit a flat hand comfortably between the lapel and your chest. A fist means it's too big. A struggle means it's too small and will pull an X-shaped crease across the button, which no alteration cures.

Cloth, and why the big number on the label is a trap

You'll see "Super 120s", "Super 150s", sometimes higher. That number describes the fineness of the yarn, not the quality of the suit. Finer yarn feels wonderful in the shop. It also creases more, resists abrasion less, and gets shiny at the seat and elbows faster.

For a suit that has to survive fifteen years, a Super 100s or 110s in a tight plain weave is a better bet than anything softer and sleeker. You want worsted wool — the yarn is combed and twisted tightly, so the surface is smooth and dense rather than fuzzy. Fuzzy weaves pill and thin.

Weight: aim for something in the 10 to 12 ounce range. Anything under about nine ounces is a summer cloth that'll wear through and won't hang properly. Anything much over twelve is a proper winter coat and you'll be uncomfortable in it three seasons out of four.

A small amount of synthetic in the blend isn't a scandal, but pure wool ages better. Polyester holds a crease well at first and then goes glassy and shiny, and once it shines there's no fixing it.

Canvas or glue

Inside the front of a jacket there's an interlining that gives the chest its shape. It's either a layer of canvas, stitched in, or a fused layer, glued on.

Fused is cheaper and it's fine for a few years. Then heat and dry cleaning break the bond down and the front of the jacket bubbles like old wallpaper. You can't repair it.

To check, pinch the fabric at the front of the jacket, a few inches below the buttonhole and away from the edge. Roll it between finger and thumb. If you can feel a distinct third layer floating between the outer cloth and the lining, it's canvassed. If it feels like one stiff board, it's fused.

Half-canvassed — canvas through the chest and lapel, fused below — is the sweet spot for most budgets and is genuinely fine for the long haul.

Choosing a cut that won't date

Fashion moves at the edges. Lapel width, trouser taper, jacket length, button stance. The middle of the road never looks wrong, and it never looks like a photograph from a particular year.

• Lapel width: roughly half the distance from your collar to your shoulder edge. Not pencil-thin, not enormous.
• Jacket length: long enough to cover your seat. The current urge to crop it will look like 2014 does now.
• Trousers: a gentle taper with a slight break on the shoe. Not stacked, not floating above the ankle.
• Color: mid-gray first. Navy second. Charcoal is more formal and reads as business rather than wedding. Black is for evening wear and undertakers and looks flat on most men in daylight.

Skip the ticket pocket, the contrast buttonhole thread, the bold lining. Those are the details you'll be embarrassed by.

Take the second pair of trousers

Trousers die first, and it isn't close. They take the friction of sitting, the seat goes shiny, the inner thigh wears through, the pockets blow out. Jackets carry on for years after the trousers have given up.

If the suit is sold with a spare pair, that spare roughly doubles the life of the whole thing. Rotate them. Don't wear one pair to death and keep the other pristine — the two will fade at different rates and stop matching.

The habits that do the work

Rest it. Never wear a suit two days running. Wool needs a day, ideally two, to recover its shape and dry out completely.

Hang it properly. A broad wooden hanger with shaped ends that supports the whole shoulder. Wire hangers put a point in the shoulder line that never fully comes out. Hang the trousers from the cuffs so gravity pulls the crease straight.

Brush it. A soft clothes brush, down the grain, after each wearing. This removes the grit that abrades fibres from the inside. It's the single most underrated thing in the entire routine.

Steam, don't clean. Dry cleaning strips the natural oils out of wool and is hard on linings and interlinings. Twice a year is plenty for a suit worn occasionally, and once a year is defensible. Between times, hang it in a steamy bathroom, spot-clean marks with a damp cloth immediately, and air it by an open window.

Empty the pockets. A wallet in the inside pocket week after week deforms the chest. Anything in the outside pockets ruins the line permanently.

Store it against moths. Breathable garment bag, cedar in the wardrobe, and don't store it dirty. Moths go for the perspiration and food traces, not the clean wool.

Budget for repairs, not replacement

Over fifteen years you'll reasonably expect to reline the sleeves, replace the seat of the trousers or have them reinforced, swap buttons, and adjust the waist at least twice. All of it costs a fraction of a new suit.

When you buy, ask the alterations tailor how much cloth is in the side seams and the trouser seat. Two inches is generous, one is normal, nothing is a problem. That allowance is what lets the suit follow you from thirty-two to thirty-six and back again.

And go to the same tailor each time. Someone who's altered your jacket twice already knows how you stand, which shoulder is lower, and how you like the trouser to sit. That relationship is worth more than the label in the collar.

Fifteen years in, the suit will have been to a wedding you were nervous at, a funeral you didn't expect, and the interview that changed your work. That's not an old suit hanging there. That's a record of the times you showed up properly.