There are a number of things engineers do to make products more manufacturable.
One is called "value engineering." Value engineering is based on the proposition that in any complex product, 80% of the customers need 20% of the features. By focusing product development, one can produce a superior product at a lower cost for the major part of a market. When a customer needs more features, sell them as options. This approach is valuable in complex electromagnetic products such as computer printers, in which the engineering is a major product cost.
To reduce a project's engineering and design costs, it is frequently "factored" into subassemblies that are designed and developed once and reused in many slightly different products. For example, a typical tape-player has a precision injection-molded tape-deck produced, assembled and tested by a small factory, and sold to numerous larger companies as a subassembly. The tooling and design expense for the tape deck is shared over many products that can look quite different. All that the other products need to have are the necessary mounting boles and electrical interface.
It's a truism that "quality is free." Very often, it costs no more to produce a product that always works, every time it comes off the assembly line. It requires a conscious effort during engineering, but it reduces the cost of waste and rework quite a bit.
Commercial quality efforts have two foci. First, to reduce the mechanical precision needed to get good performance. Second, to control all manufacturing operations to assure that every part and assembly are within tolerance.
Statistical process controls on manufacturing usually proceed by randomly sampling and testing a fraction of the output. Variances of critical tolerances are continuously tracked, and manufacturing processes are corrected before bad parts can be produced.
A valuable process to perform on a whole consumer product is called the "shake and bake." Every so often, a whole product is mounted on a shake table in an environmental oven, and operated under increasing vibration, temperatures and humidity until it fails. This finds many unanticipated weaknesses in a product. Another related technique is to operate samples of products till they fail. Generally the data is used to drive engineering and manufacturing process improvements. Often quite simple changes can dramatically improve product service, such as changing to mold-resistant paint, or adding lock-washed placement to the training for new assembly personnel.
Many organizations use statistical process control to bring the organization to Six Sigma levels of quality. In a six sigma organization, every item that creates customer value or disasstisfaction is controlled to assure that the total number of failures are beyond the sixth sigma of likelihood in a normal distribution of customers - setting a standard for failure of fewer than four parts in one million. Items controlled often include clerical tasks such as order-entry, as well as conventional manufacturing tasks.
Another engineering discipline is "producibility." Quite frequently, manufactured products have unnecessary precision, production operations or parts. Simple redesign can eliminate these, lowering costs and increasing manufacturability, reliability and profits.
For example, Russian liquid-fuel rocket motors are intentionally designed to permit ugly (though leak-free) welding, to eliminate grinding and finishing operations that do not help the motor function better.
Some Japanese disk brakes have parts toleranced to three millimeters, an easy-to-meet precision. When combined with crude statistical process controls, this assures that less than one in a million parts will fail to fit.
Many vehicle manufacturers have active programs to reduce the numbers and types of fasteners in their product, to reduce inventory, tooling and assembly costs.
Another producibility technique is "near net shape" forming. Often a premium forming process can eliminate hundreds of low-precision machining or drilling steps. Precision transfer stamping can quickly produce hundreds of high quality parts from generic rolls of steel and aluminum. Die casting is used to produce metal parts from aluminum or sturdy tin alloys (they're often about as strong as mild steels). Plastic injection molding is a powerful technique, especially if the part's special properties are supplemented with inserts of brass or steel.
When a product incorporates a computer, it replaces many parts with software that fits into a single light-weight, low-power memory part or microcontroller. As computers grow faster, digital signal processing software is beginning to replace many analog electronic circuits for audio and sometimes radio frequency processing.
On some printed circuit boards (itself a producibility technique), the conductors are intentionally sized to act as delay lines, resistors and inductors to reduce the parts count. An important recent innovation was to eliminate the leads of "surface mounted" components. At one stroke, this eliminated the need to drill most holes in a printed cricuit board, as well as clip off the leads after soldering.
In Japan (the land where manufacturing engineers are most valued), it is a standard process to design printed circuit boards of inexpensive phenolic resin and paper, and reduce the number of copper layers to one or two to lower costs without harming specifications.