1) Cross check the compatibility of your parts beforehand (especially Mobo/CPU/RAM)
2) Check energy requirements of your setup (online tools can do this) and make sure you choose a PSU that can handle the requirements. I usually add on an extra 25% to the number for safety/future proofing.
3) Usually easier to mount CPU/RAM before installing your motherboard, especially if case is tight on space. Depending on the type of CPU cooler you have, you could also install this beforehand.
4) When mounting a motherboard, make sure you haven't installed any unnecessary mounting screws on the case that can touch parts of your motherboard which aren't meant for mounting. Had a mate who rushed this and overlooked it, leading to a short circuited motherboard soon after.
5) Always check your CPU cooler is mounted properly, sometimes it can be hard to tell with more fiddly coolers. Nothing worse then having temperature issues taking a toll on the CPU for something as silly as this.
6) If your case doors won't shut properly without a silly amount of force then reconsider your cable management, no reason to put additional stress on the chassis/parts.
7) Don't ever force a CPU into it's slot. If it doesn't go in seamlessly with no effort, then chances are it's orientation or even chipset is incorrect.
8) Hang on to spare parts/cables/etc you never know when they will come in handy
9) Any part that produces heat should be exposed to some sort of airflow. Last thing you want is a hotpocket
10) When setting up fans be sure to think about your cooling strategy. Don't have heat recirculating within the case
11) Avoid using ducktape for cable management, zip ties are easier to manage and won't leave adhesive over your cables (especially after a long time of use in a hot system)
12) Whilst tight cable management is good, avoid having cables so tight that there is noticeable tension on parts/connectors. Over time this can cause issues. Leave a bit of slack on the cables
