This will be about basic smooth leather shoe and boot care for traditional non-sneaker shoes. Some of this may apply to sneakers, but sneaker leather is usually different than good quality dress/casual/boot leather, often having plastic coatings on the leather. (There are "dress shoes" with plastic coatings as well, those aren't good quality for the purposes of this post. If polish doesn't stick or conditioner doesn't soak in a bit it's probably coated)
TLDR version: Don't wear the same shoes every day, once or twice a year put some Bick 4 on, let it dry and wipe it off. That's enough to maximise longevity, the rest is cosmetic.
There are different levels of care. There isn't all that much you can do for most soles. For leather soles, sole guards (thin rubber sheets glued to the soles by a cobbler) wear longer...or at least switch the wear to an easily replaceable rubber layer. If the soles are durable or worth replacing keeping the uppers in good shape makes more sense. Even minimal care will help with looks. Note, the look I'm going for is "well cared for" rather than "first time wearing". If you are going to replace soles and heels, pay attention to the layers. If you catch the wear at the first layer of sole or heel the job is easier and cheaper, the second layer (in most shoes) somewhat more expensive. If the wear gets to the welt the job becomes much more difficult and expensive, and likely not worth the cost.
Preventative: Number one is rotate your shoes--Give your shoes or boots time to thoroughly dry out between wears. Alternating between two pairs will give more than twice the life of the uppers compared to wearing one pair daily. Use shoe trees for storing dressier shoes to reduce wrinkling, curling and creasing. Trees don't have to be fancy. Cedar trees will absorb moisture and help the shoes dry faster but plastic or metal will still help hold the shape of the shoes. Trees also make polishing easier.
Keep your shoes reasonably clean. Brush dirt and dust with a horsehair brush, use a damp rag for salt or dirt that the brush won't get. Don't let salt sit on your shoes. I often see diluted vinegar recommended for salt stains, haven't needed it yet since hearing of it.
A good conditioner, applied about twice a year under normal use. Shoes that are in severe conditions will need conditioning more often, but unworn shoes still need periodic conditioning--If the leather is stiff, it probably needs to be conditioned before wearing to prevent cracking. Bick 4 is probably the most widely recommended all purpose conditioner among shoe snobs--it protects well without darkening most leather, it is relatively inexpensive but still used by some of the best cobblers. Boot oil, mink oil or neatsfoot oil give more protection in severe use but are likely to darken the leather and prevent a high shine, and over-conditioning is possible. Oils are usually limited to workboots (and similar styles of fashion boots) where durability is more important than shine.
To use Bick 4--Apply with whatever--sometimes I use fingers, a rag, an old toothbrush gets into crevices better. Let it soak in for 15 minutes or so. If it soaks in instantly, add more. After it dries brush it with a horsehair shoe brush or a soft cloth for a slight shine.
If you want more gloss, a cream polish in addition to conditioner. Venetian Shoe cream is a good neutral cream--another inexpensive product used by some very good cobblers. Neutrals are great when you don't have color damage, and especially if you have different colors of leather--a natural welt on a black shoe for example. (If you're wearing spectators...you don't need advice from me) Creams will give more shine than Bick but not as much as a wax. If there's color damage I use a colored cream, sometimes only where the scuff is depending on how closely the polish matches the shoe. Saphir has the best reputation, Tarrago and Kelly's are also brands I'm happy with. I use mostly Tarrago because it's available at a local shoe store at about half the price of similar quality polish from Amazon. Apply with an applicator brush (a toothbrush works) or cloth, let dry, buff with a soft cloth or horsehair shoe buffing brush.
For a high gloss, a wax based polish. Kiwi is a low quality example of a wax polish. I use Allen Edmonds for colored wax because it has a good reputation and it was on sale. Saphir for neutral--I don't need much, and it works with all colors. I like to let waxes sit overnight if possible especially if trying to fix scuffs and scratches. Wax polish will build up and crack if over-applied. Cracking is mostly a problem with mirror shined shoes...which I don't do. I concentrate on the toes and heel area, avoiding wax buildup on flexible parts where it might crack. You don't always need more polish to get a shine, just buff what's already there. You also may not need a colored polish to fix a scuff, sometimes just some conditioner or neutral polish will blend in the damage.
Not all leather shoes can be realistically polished. Oily leather boots are difficult to make shiny. Conditioner still helps with oily leather, and a pigmented polish can help with uneven color. Plastic coated leather is essentially maintenance free...not that it doesn't need maintenance, but rather it won't do much good.
Avoid getting dark polish on lighter colored areas--you don't usually have to polish with color right to the edge. An exception is for decorative perforations--some people will use a darker polish wiped off immediately to highlight the perforations. Stitching may or may not absorb the color.
For buffing I have a brush for black, and a brush for everything else. Applicators are specific to that type and color of polish, dark brown and light brown use different brushes, cream and wax are also different. For oddball colors (I've got one pair of navy blue loafers...that I've polished but never worn) I'll usually use a clean spot on a rag.