Buying climbing shoes that fit just the way you want is an arduous process. Unlike running/tennis shoes, you can't just walk into the store, select the size that usually fits in your street shoes and be done with it. Instead, the fit of a climbing shoe is a very personal and finicky thing.
What type of climbing are you using these for?
Consider what type of climbing you prefer. Do you mostly boulder or crack climb? Do you love sport climbing or would you rather multi-pitch? Each type of climbing is better done with a more specialized shoe and shoe size.
Bouldering and Sport Climbing
If you are more into bouldering and overhung sport climbs, then an aggressive, down-turned shoe is going to work best. This design puts more power into the toe. However, to get the most of of these shoes you need a very precise and usually tight fit. So the trade off is that these shoes are rarely comfortable for more than a climb or two. They also tend to be expensive, not smear that well on slabs, and be painful.
Trad climbing and crack climbing
If you climb cracks or multi-pitch, you want a bigger, flatter, more comfortable shoe. Generally you want a stiffer sole that can stand on edges even when the shoe is fitted more comfortably. An unlined upper may give you more comfort, but it might also stretch and have the shoe become sloppy. Take that into consideration when sizing.
Gym Climbing and Starting Out
If you are just starting out then you probably want a more comfortable, versatile, all-around shoe. Don't go for the aggressive down-turned shoes. Go for something that is not too tight and does not cost much. Shoes that close with Velcro are great because you can get them on and off fast. But a comfortable pair of lace-up shoes can be great as well.
The below image by La Sportiva shows a good overview of different shoe styles. Most climbing shoe companies have shoe designs that span a similar range.
There are two schools of thought with climbing shoes: that your shoes should be extremely tight to the point of pain, or that your shoes should be bigger and more comfortable. Really tight shoes are great for sport climbing and bouldering where sensitivity on edges, tiny foot chips, and smears can be the difference between sending and whipping. Also, on sport climbs you tend to only wear your shoes for the length of one pitch so the tightness and curled toes can be bearable. However, don't size your shoes so small that the discomfort is excruciating and makes you not want to use your feet.
On multi-pitch climbs shoes are worn longer and really tight fitting shoes can actually make you climb worse because you are less likely to want to put any pressure on your aching toes. So in that situation I would recommend bigger shoes. Also, for crack climbing, when you jam your feet into the crack and tweak them around, shoes a bit bigger are actually an asset.
When shopping for shoes decide beforehand what you plan on climbing most with these shoes and size them accordingly. If the shoe is unlined be aware that it may stretch and become more sloppy over time. In general, we find that just going a little bigger than tight can make a shoe 50 percent more comfortable while only losing 5-10 percent of the performance.
More and more shoes are coming on the market with an aggressive, down-turned toe. Keep in mind that the more down-turned a shoe, the more volume is left for your toes to bunch in the front. If you buy a down-turned shoe really big, you are likely to have a lot of extra space above your toes, and often this bunches up uncomfortably while you are climbing. The more aggressive the shoe, the better it is to go for a really snug fit – not painful but snug.
Now that you have decided how tight your shoes should be, factor in how much the shoes will stretch after you buy them. Most shoes don't keep the same fit that they have in the store.
Leather shoes stretch a decent amount, so prepare yourself when buying them. The exception: leather shoes that are lined. For example, Mythos are unlined leather shoes and stretch quite a bit after a lot of use. The Miura are lined leather shoes and do not stretch much at all, but they do mold to your feet. Synthetic shoes, like 5.10 Anasazi's and most Evolv shoes, do not stretch much so you can buy those pretty true to size.
Pay attention to what kind of shoes you are buying (and the design and materials) so you know what to expect once the shoes break in.
Brand and sizing
As a rule of thumb, 5.10 and Evolv shoes fit much smaller than European brands such as La Sportiva or Scarpa. They make it so you can buy your street shoe size now. Don't expect to wear the same size shoe across the board, but learn how each brand fits your uniquely shaped foot.
Laces, Velcro or Slipper
There are three main types of tightening system: Velcro strap(s), lace-up and slipper (usually elastic). Lace-up shoes generally give the most precise, snug, and secure fit but take a while to get on and off. Velcro shoes go on and off quickly and many newer Velcro shoes tighten almost as precisely as lace-ups. The Velcro design is our favorite because we can get them on and off fast whether it is a really tight shoe for gym, sport or bouldering or a trad climbing shoe where we just want a little relief at a multi-pitch belay. Slippers are generally the most comfortable and sensitive shoes. They are usually unlined, which means they stretch and become even more comfortable but less precise over time. They often make great gym shoes because they give such great sensitivity while pasting your foot on big holds on steep walls. Then, after your climb, you can usually have them off in the time it takes to lower to the ground.
Tip: Store Velcro shoes with the Velcro closed. It keeps the shape of the shoe better, makes it less likely for muck to get in the Velcro, and therefore extends the life of the Velcro and the shoe.
Rubber, more than any other factor in climbing shoes, is all about personal preference. People have debated forever what is the best. There is one general rule: when looking at the stickiness of a climbing shoe it can be measured in a continuum, where one end measures stickiness and the other measures durability. The closer you get to sticky, the less durable the rubber and vice-versa. Rubber that is really soft and sticky is likely to wear out fast, create little "fish scales," and generally melt off tiny vertical edges if you are standing on them for more than 20 seconds. A harder rubber is more durable and will hold an edge longer, but also will not stick to the rock as well if you lightly paste your foot on a hold. So there is a trade off. In general, we like our high performance shoes as soft and sticky as possible unless we are climbing edgey face routes. For entry level shoes or all-day trad shoes, a harder and more durable rubber maybe the ticket.