Essentially; know your system, know your data, know your bottlenecks, know the plateaus, know your growth patterns, and keep an eye on future technology.

You have limited CPU, limited disk space, limited disk IO, limited memory (note: taking more space on disk also takes more space in RAM), limited time, limited concurrency, and limited humans. All these factors must be balanced.

Know your bottlenecks, but don't waste anything. If you want an 8k row tally table, use smallint, not int. If you have a date dimension table that covers SMALLDATETIME, every single day can be represented by a unique SMALLINT value, starting at -32768 for Jan 1, 1900.

Note: It's not uncommon for a 146GB 15k FC "upgrade" disk to cost right around $1000... each. Not counting the tray cost. Not counting the cabinet cost. Not counting license costs for the SAN to be allowed to see it. Not counting parity/mirror disk costs. Not counting hot spare costs. Not counting cold spare costs. Not counting electricity use. Not counting heat generation.

Plateaus are critical: Adding another couple drives is "easy" and "cheap"... until you're full, at which time you may need a new tray, which needs a new cabinet, which needs more floor space in the server room, and which needs a new electrical circuit, which needs a new enterprise UPS, which needs a new generator... and the additional heat load needs a new air conditioning unit. Did we mention the fire suppression system, too?

Now, if SQL Server gave us a comprehensive range of integer datatypes, we'd be a lot better off; we should have both signed and unsigned 8, 16, 32, and 64 bit integers. We don't.