Your underwear may well be your most important article of clothing, and cutting yourself short in this department has far-reaching implications. Few salespeople know very much about fitting underwear, and fewer still will talk about it, so it's a trial-and-error proposition that a lot of people never resolve. The many brands are so different that fitting becomes much more than sizing. The quality of the brand and the cut of the article are as important to comfort and utility as the size itself.
Decide whether you are committed to briefs, boxer briefs or boxers. There are variations, such as bikini briefs that are simply lower-rise but basically fit the same as briefs. Many (maybe even most) men base their grooming and clothing decisions for their entire lives on arbitrary factors, i.e., what your father wore or what your mother bought you when you were a child. You should at some point buy at least one pair of each style you haven't worn so you can make up your own mind. Some people like to wear two or all three styles (including the low-cut briefs), depending on the situation and activity.
Select briefs with the same waist size as your trousers. Thus, if you have a 90 cm (36 inch) waist, opt for a 90 cm (36 inch) brief. The advantage of the brief is increased support, so you should fit it somewhat tightly across the hips and thighs. If this is uncomfortable, you should probably consider wearing boxers. Note: if you have an odd waist size that you are able to fit, you should select the next size up. Briefs only come in even-numbered inch sizes; thus, a 87.5 cm (35 inch) waist would take a size 36 brief.
Select boxer briefs with a bit more flexibility than briefs. The extra length on the leg creates a completely different feel and fit than briefs. In fact, the fit for boxer briefs is closer to boxers than briefs. But start by sizing the same as your trousers if you have been wearing briefs. If you find the leg too binding, try the next (even) size up. If you have been wearing regular boxers, you should probably try the same size as you wear in boxers.
Select regular straight-leg boxers the next size up from your trousers. Thus, a 90 cm (36 inch) waist would call for a 38 boxer. The advantage of the boxer is comfort and freedom, rather than support. Opting for a tighter fit compromises that benefit, so it doesn't make sense. Most people will find that going up a size improves comfort and flexibility of movement.
Consider how tightly you prefer to wear your trousers. If you like tight-fitting jeans, you may find that briefs better serve your needs. If you wear hip-hugging low-rise trousers, the bikini brief may work better. Relatively loosefitting dress trousers are more compatible with boxers, but you can still wear briefs if you prefer the feeling, the enhanced support and the tighter fit.
When buying boxers, look at the construction of the front opening or slot. It should provide maximum overlapping of fabric, and the slot should not extend all the way down to the crotch seam. Some brands, such as Nordstrom, will have buttons, which are not very practical, but are still better than the overlong slot found on too many less expensive brands.
Look for fullness in the legs and seat. A good boxer will have tapered legs and a shallow seat that fits closer to your buttocks. This is important, because a full seat and wide leg will cause the underwear to ride up into your crotch when you sit, and you will usually feel uncomfortable when you stand. Always having to adjust and pull down your underwear is awkward and often distracting, but you don't have much choice when your boxers are continually jammed up high against your hips and upper buttocks. If the leg is well-tapered but the seat is still too full, they will still ride up, but perhaps not quite as far.
Feel the material weight. Thin underwear not only wears out too soon, but never seems to fit as well as more substantial material. Nordstrom isn't bad in this regard -- in fact, their material may be too heavy. Calvin Klein is probably the best in terms of the balance between overall design and material weight and quality, and they last even longer than the Nordstrom brand.
Buy only 100 per cent cotton -- never any blend or synthetic material. Silk feels nice, but doesn't clean and wear with as much longevity. You are much better off having seven or eight pairs of underwear you really like that cost you £9 each than 50 pairs you bought in a sale or that never feel quite right. Some brands like Nordstrom will offer their seasonal designs and gaudy prints at substantial discounts between seasons -- other than the colour and design, it is the same underwear they normally stock.
Paying cut-rate prices for underwear usually amounts to throwing away money.