Step-1 What are my real power needs acording to the hardware I have, also taking into acount near future changes and possible upgrades?
The power comsumption of all hardware, and puting emphasis on +12V rail (Total Combined Continuous Power/Current Available on the +12V Rails) cause it will be the most demanded rail.
And remember the formula Amps x 12=wats.(valid only on single rail designs). For instance on this Corsair unit (100.4A x 12 = 1204.8W)
On double rails or more designs its a diferent matter, for instance on this ASUS dual rail desing
We have +12v1=18A and +12v2=18A 22A MAX.It means that the max continuous current combined available would be 22A, for instance if on +12v1=12A, then the other can deliver only 10A MAX,the summ of them is allways 22A=264W
Some users just summ the rails and that is really a huge mistake, for instance on ASUS above 18A+18A=36A then as a result 36A x 12=432W. And thats not the real power of that PSU.
Step-2 The quality of PSU is really important,so you must look for:
-Good Branded(never generic PSU)and look for the real manufacturer,cause sometimes are rebranded PSUs,for instance Corsair,Antec,XFX etc, most of them use SEASONIC topologies,and SEASONIC is one of the best OEM on the market from all times
-Voltage regulations( for instance the 5% on +12v as the max up or down)
-Noise and ripple levels (the lower the better)
-PFC active or pasive(active is the best)
-Efficiency (the higher the better)see table
-there are others things to take into account, but if you care about above you would be safe.
Step-3 Once decided the power needed you have to see the physical conditions you got inside your case, cause based on that you will chose from 3 diferents kind of PSUs:
Fully-Modular ( you can pull out all the cables).
Semi-Modular( you can pull out part of the cables).
Non-Modular (you can not pull out any of the cables).
So, for instance if you dont have cable management on the case you will need a modular desing PSU for better cable suiting(you wont like an case interior full of cables blocking all your case ventilation).
And if you have a case with good cable management you can use a non modular PSU(saving some money).
The best way would be good cable management and modular PSU desing (but more spensive too).
In my opinion a semi-modular PSU covers almost all needs.
Step-4 When you chose an specific PSU that you think is the one you need,you can look for some proffetionals reviews, to see if everything is ok,and compare results to others PSUs(they make a several sintetic and real world tests that can reveal any detail and the real power of the PSU).
Step-5 try to be 100W over the recommended PSU minimal power needed to have more headroom to be really safe, If you follow the things above you wont have problems on the future.