The best way, and most common is the use of cable sleeving. There are many types of sleeving, some very easy to install, some more difficult. It seems that the most commonly used product available for sleeving now is "Para-cord". It has a few desirable features, remains supple and more color possibilities. It takes some practice to get proficient in sleeving (I'd suggest practicing on a fan or two first). There is also split-side type sleeving (can even be found at some automotive supply stores) which just sorta clamps around the cables (it can look cheesy though). Last, there are sleeving kits such as these http://www.frozencpu.com/cat/l2/g35/c183/list/p1/SleevingHeatshrink-Sleeving_Kits.html YouTube can help here...
The best way, and most common is the use of cable sleeving. There are many types of sleeving, some very easy to install, some more difficult. It seems that the most commonly used product available for sleeving now is "Para-cord". It has a few desirable features, remains supple and more color possibilities. It takes some practice to get proficient in sleeving (I'd suggest practicing on a fan or two first). There is also split-side type sleeving (can even be found at some automotive supply stores) which just sorta clamps around the cables (it can look cheesy though). Last, there are sleeving kits such as these http://www.frozencpu.com/cat/l2/g35/c183/list/p1/SleevingHeatshrink-Sleeving_Kits.html YouTube can help here https://www.youtube.com/results?search_query=psu+sleeving+tutorial
Agree with C12Friedman sleeving is most common and probably the best option, not just in PSUs but also in DCUs (driver console units) like this one https://www.qconnect.com.au/CustomCables/CustomProjects-456/