Undie 3....

Never leave your house and you can survive on a piece of underwear for three days. pandafromars

And even if you leave your house, you can easily survive two days, aerate them, and wear them again. Travel for a month with a backpack teach you things, I swear. tia_avende_alantin33

Logistics. 

Here's my list: 3-4 plain shirts, 1 semi-formal jacket, 1 jeans and 1 slacks, 1 shorts, 3 socks, 3 underwear, 1 leather shoes and 1 jogging shoes. It will be well under 7 kg So you still have room for your laptop, gadgets, etc. Heli456

The Thread. 

Needle and thread of the colors you'll be wearing visibly.

Take little to no space, and no scissors needed, just be careful when breaking the thread.

A missing button or a hole sometimes make a difference. The_spanish_ivan

UPS Rescue. 

I shipped my suit and a return label to the hotel I was booked at to my attention, and called ahead to let them know I would be arriving within a few days to pick it up if they wouldn't mind holding it (call first). Then when I left, I left it behind the desk with them and scheduled a UPS pickup. RogerPackinrod

Dual Use.

Dual use undershirts. Seek out locations and hotels with a laundry service. Go as casual as possible with your pants while focusing on more formal shirts and jackets as tops. wasstronger

An Extra Closet. 

Garment bag for your formal wear and stick it in the overhead. Fit whatever else you can in a backpack that fits under your seat.

If you fly first class, they'll usually have a small cupboard where you can hang your jacket. babygrenade

They usually offer this to coach passengers. I've never been turned down when I've asked to hang my suit in the suit closet. just-signed-up

Spaced Bags. 

The last time I needed to travel light, I was able to fit a week's worth of clothes, including ski outerwear, into a carry on by using vacuum space bags. Not going to work for every situation and type of clothing but solved my problem beautifully. jsbt1977

Rent the Runway....

I usually will use rent the runway, which is just women's clothes but they have similar sites for suits and men's clothing. But you can get a whole outfit delivered to where you're staying for the formal occasion and pack for just your vacation. It's not necessarily the cheapest option, but my style for weddings has stepped up greatly and I haven't bought anything new recently. FatLadiez

The Overdress....

Travel in your heaviest clothes, but comfy. If you need a suit or dress make sure it's thin if you're going somewhere hot. Then pack maybe 1/2 things of swimwear and wash and reuse clothes throughout the stay. twentyonepilotsksk

A Studied Method. 

If you can afford it, rent the formal outfit or borrow one from a friend headed to the same formal occasion

Merino wool tends to remain odor free and you don't need to iron it — wears well for many days. Lots of brands sell merino wool items for travelers. You can dress it up too.

Linen is great for tropical climates — it isn't anti odor but wicks away sweat so you stink less and doesn't look too bad even when wrinkled — 2-3 shirts and bottoms can be rotated over a week. Uniqlo makes great linen pants and tops.

A study has shown that synthetic fibers (polyester) might smell more than natural (cotton) fibers

https://www.realmenrealstyle.com/fabric-types-affect-odor/ moonandmilk

Have fun!!!

I like to pack in one color scheme, so that can all be mixed and matched. This also lets you pack only 1 pair of shoes (wear the other pair you plan on using).

For the formalwear, you'll have to suck it up a bit, but that should help with minimizing space for the vacation days after.

Also, pick your travel outfit intentionally, so that it's one you can rewear on the trip or at least on the plane back home.

Have fun! whocanpickone

Don't Eat it Up. 

Shoes eat up a lot of space so I limit myself to 1 pair of comfy shoes that are acceptable to wear on formal occasions like weddings or meetings while making sure they don't hurt like hell when strolling around town or the beach.

You'll need a few of your best shirts that work well with slacks and jeans. Here's a guide that might help you decide on essentials to bring along.

I also have 1 cool looking jacket that almost looks like a suit and seem to always get away wearing it in meetings and formal days at work. Hope this helps. MrDavidStevens

"in case you wanna wear it"

One overhead suitcase, one moderately sizeable backpack to go under the seat in front of you. Don't bring anything you're not certain that you're gonna use. Don't bring any maybe-outfits "in case you wanna wear it". Don't bring a book that you might read. If it's a maybe, it's a no. Pack more socks and underwear than shirts and pants/shorts. Pants and shirts can be reworn; underwear and socks can be too, but that's gross, so bring more.

I know there's the old adage "better to have it and not need it than to need it and not have it," but not when you're trying to pack light. Chances are, if you have enough money to be traveling/vacationing, you have enough disposable income for you to buy anything that you forgot (or anything that you decidedly didn't bring that you ended up needing after all). Reddit

DO NOT ROLL YOUR FORMAL CLOTHES!

I am not what anyone would consider a "light packer" but I pride myself on fitting obscene amounts of clothing into my suitcase when I travel. DO NOT ROLL YOUR FORMAL CLOTHES. I've seen this mentioned a few times and while it works fine for things like t-shirts it doesn't work well for bulky and stiff items like a suit jacket. The most effective way I have found for packing formal wear is to fold the sleeves over the body and lay it as flat as possible either in the bottom of your suitcase or the top. If packed well you can easily fit a weeks worth of clothing in a carry on, including your formal wear. Shakith

Ship to You.

If you can afford to do it, and it could get pricey, you can ship things to yourself in advance to any post office in the US. It is called general delivery . I had no idea this existed until my 20s and have only ever used it once. a116jxb

The Gussie Method....

Been traveling quite a bit in the last few years, for up to 4 weeks at times. Found this video earlier this year and I now swear by it, or at least a close variation based on my own packing needs. I never travel with more than a carry-on for above the seat and a backpack for under. I present you: The Gussie Method https://www.youtube.com/watch?v=PDn9l20NlWw&t=5s Luckybird1

 "just in case I want to look cute."

Stop packing those outfits that you never wear "just in case I want to look cute." You will either put them on, then immediately change into something more comfortable, or they won't even leave your suitcase. Shoes too, one comfy pair for walking, and one formal pair that is also comfortable.

Same with makeup/toiletries. Pack your every day things and MAYBE one "special occasion" lipstick. You don't need that fancy palette that you never use or those 3 hair styling tools that take forever! recklesschopchop

Checklist. 

1.Always use versatile clothing and shoes. Shoes and jackets have to be limited to 1 pair/piece.

2.Always, and I repeat, always go with wet wipes. Your suit, shoe, etc. can be stained, and if you immediately rub it with a makeup remover wipe (the micellar water ones) it'll likely go away.

3.Take a handag that doesn't look bad with formal clothes, and a small overhead luggage (preferably the hard plastic ones, not the fabric ones, as those maintain the structure you packed best).

4.Leave space for things you might get, take only plain clothes, eg 1 pair of jeans+t-shirts. Always take an extra set of underwear.

5. Check what the hotel/airbnb you stay in has. If they have towels and shower gel, you only need a comb, razor (optional), toothbrush and small toothpaste for toiletries. This often frees up space. DrLilyPaddy

Thanks Wifey. 

A pair of shoes can contain around 2 pair of socks and 2 pair of underwear inside.

Don't fold any cloths roll it up. takes up a lot less space.

Source: I got my wife to pack all her stuff in one (!!) bag. NicolajN

All the Right Things....

For the occasion, into the baggage: One shirt, one pair of formal pants, formal shoes, a tie.

Then for N days of holidays, into baggage: N/2 t-shirts. N pairs of underpants, N pairs of socks.

Misc stuff: a towel, pair of shower slippers.

Then you wear: 1 pair of jeans, 1 pair comfy/sporty shoes, t-shirt, sweater/polar, coat/windcatcher.

I always fit all of the above stuff into one backpack along with the laptop when I have to travel for work for a week or two.

PS. On the very formal occasions where you also need a jacket and/or - I travel in a suit, as it's very hard to fold properly.

PPS. Check the weather forecast for your destination, adjust accordingly. swistak84