Does the fan on the PSU start up every time? if not - suspect the power cable/connection to psu
next check the dumb but easy to overlook:
is the psu switched on when you press the power switch?
is the mobo shorted against the case at all - probably worth removing everything from the case and bench testing on the mobo box before moving to case
If all looks ok
- does the Mobo show any indication of power? - led, cpu cooler fan, etc - if not: first make sure the 'power on' connector is properly connected between mobo and case, if it seems ok try shorting the 'power on' connections directly on the mobo - just touch a screwdriver across them. if that works its a faulty power switch. if it doesn't check the 6/8 pin connector to the cpu is seated properly
if still dead take all the disks/pci cards and ram out (leave the cpu) and run through the same tests, if that works add things back one by one and test each time