Design for Manufacturing (DFM) is the habit of designing a part so it's easy and cheap to make, not just functional. The cheapest features are the ones the process makes naturally; every time you fight the process you add cost, time and scrap. A few principles carry most of the value.
Give molded and cast parts draft
Anything pulled from a mold or die — injection-molded plastic, die castings, forgings — needs a draft angle (typically 1–2°) on every wall parallel to the pull direction, or the part won't release cleanly. It's nearly free to add in CAD and painful to add later.
Round internal corners, keep walls uniform
Sharp internal corners are stress raisers and, in machining, simply can't be cut by a round tool — so give them a radius (at least ⅓ of the depth). Keep wall thickness uniform: thick and thin sections cool and shrink at different rates, causing sinks, voids and warping in molding and casting.
Don't over-tolerance
Tolerance is the biggest hidden cost driver on a drawing. Every tightened tenth needs a better process, more inspection and more scrap. Use general tolerances (ISO 2768) everywhere and reserve tight callouts for the one or two features that actually mate or seal — and use stack-up analysis to prove they need it.
Design for the process
Machining: standard tool sizes, avoid deep narrow pockets, allow tool access. Sheet metal: uniform bend radii and generous flanges (see the sheet-metal guide). 3D printing: mind overhangs and supports. Fasteners: minimize part count and use standard hardware. The best part is the one you don't have to make at all — combine features and eliminate assembly steps.