I think this is the most important thing that I learned in all my years making things.

You see, things break. Sometimes they wear off or become obsolete, and sometimes they arrive already broken. And from time to time you make a mistake and release the blue smoke from them. But it's not just that. Often you just want to remove a part to see how things work without it, or temporarily switch it for another part. Or scrap an old project and recover parts from it.

Whatever the reason is, it's all much easier when you can simply unplug and/or unbolt it. Therefore:

• Use nuts and bolts, screws or other reusable fasteners. Avoid gluing things together as much as possible, unless it's some weak glue like adhesive tape or hot glue, which you can easily remove. Even then, bolts are much better.
  You can use nylon or acrylic bolts if you care about the weight.
• Use plugs, preferably on both ends of the wires. Avoid soldering wires directly to the boards, or soldering boards together.
• If possible, make parts individually removable. Avoid situations where you have to remove one thing to get access to another thing.
• Use off-the-shelf parts as much as possible, and avoid customization. Any modifications you make will need to be repeated when you are replacing the part, and may make the part unusable in other projects.
• Design in modular way, so that you can disconnect and test subsystems individually.
• Try to follow existing standards and established practices when designing your plugs and mount points. Try to use as few different kinds of plugs and fasteners as possible, so that you can later swap elements. Also follow this across different projects.
• However, don't use standard connectors in non-standard ways. Do not use an audio jack to provide power, or an USB plug for I²C. If you absolutely have to, make sure that connecting something there by mistake won't result in a catastrophe.
• Try to anticipate alternate uses. Use footprints that can accommodate several versions of the component, place headers so that you can use either straight or angled ones, include additional mounting holes, etc.

This will not only let you fix your projects when they break, it will also allow you to diagnose the problems in the first place, and will make your projects easier to reproduce by other people or yourself.