Most packing advice is preference; a few things genuinely affect the trip.
What is hard to replace
Prescription medication, spectacles and documents.
Which should be duplicated or carried in hand luggage.
Airline restrictions
Weight, dimensions and prohibited items.
Which vary by carrier and are enforced.
Layering
Adapting to conditions rather than packing for each.
What can be bought there
Almost everything, in most destinations.
Which people consistently forget.
The short list that genuinely matters
Medication in hand luggage with a prescription copy, documents duplicated separately, and anything with a specific fitting or prescription that cannot be replaced quickly.
Which is a handful of items.
Everything else can be bought at the destination in almost every country, and packing as though it cannot is the main cause of unnecessary weight.
Hand luggage strategy
A change of clothes and essentials in case checked bags are delayed.
Which happens regularly.
Restrictions
Liquids, batteries and prohibited items.
Which airlines publish and vary on.
Weight allowances
Charges for exceeding being substantial.
A general note
Medication rules differ by country; some common medicines are controlled elsewhere.
What actually makes a trip go well
Very little of it is the things travel content concentrates on. It is not the property, the itinerary or the equipment. It is preparation on a small number of specific points, a pace that leaves room for things to go wrong, and enough humility to ask people who live there.
The trips that go badly are usually the ones where something checkable was not checked: an entry requirement, a health preparation, a transport connection, or the assumption that a place would be like somewhere else. All of those are answerable in advance and none of them takes long.
The preparation that repays itself
Entry requirements from the destination’s own authority. Travel health advice with enough lead time for vaccinations. A realistic daily budget including local transport, food and activities. Twenty words of the language. Somebody at home who knows roughly where you are.
That is an hour or two spread over the weeks before departure, and between them those five things prevent most of the situations that turn an ordinary problem into a serious one.
The travel that seems to be worth most
Longer stays in fewer places, some contact with people who live there rather than only with people paid to serve you, and a willingness to be uncomfortable occasionally. That is what the research on attitude change points at, and it is also what people describe when they talk about trips that mattered to them.
None of it requires money. Some of the most valuable travel is the cheapest, which is not what the industry has an interest in saying.
A general note
Entry requirements, health advice, safety conditions and local law differ enormously by destination and by nationality, and they change. Official sources for your own nationality and your specific destination are the only reliable reference, and nothing here is a substitute for checking them.
What travel writing tends to leave out
Most of it is written to inspire rather than to inform, which means it emphasises the extraordinary and skips the logistics. That is a reasonable commercial choice and it produces a systematic gap: the things that actually determine whether a trip works are rarely the things that get written about.
Entry requirements, health preparation, realistic budgeting, transport connections and what a place is actually like to be in on an ordinary Tuesday are all considerably more consequential than any recommendation, and considerably less photogenic.
On the destinations themselves
Almost every place is more ordinary and more interesting than its reputation. The reputation is built from a handful of images that a great many people have now seen, and arriving expecting them produces disappointment that has nothing to do with the place.
The parts of a destination that people remember afterwards are generally not the ones they went for, which is an argument for leaving room in an itinerary rather than filling it.
Where to get reliable information
Foreign ministries publish entry requirements and safety advice for their own nationals. Travel health services publish destination-specific medical guidance. National tourist boards publish practical information without a commercial interest in any particular property.
All three are free, current and specific to where you are actually going, which is more than can be said for most of what a search returns.
One habit worth adopting
Ask someone who lives there. Accommodation staff, drivers, shopkeepers and people at the next table know things no guide contains, and most of them are pleased to be asked.
It is also the part of travelling that people remember, which is a reasonable argument for it beyond the practical one.
A final caveat
Conditions, requirements and local circumstances change, and anything written at a point in time carries that date. Official sources for your nationality and destination are the reliable reference before any trip.