unplug all accessories at the motherboard, including hard drives,cdroms keyboards ,mice,hubs etc.
1 stick of ram. try it in different slots. try a known good stick of ram (pull from a working box)
no pci/e cards (except video if no onboard video)
no internal front usb2/3 cables (i had a front usb port (in the case) that would randomly short and not let it post, one time it did post and bios flashed a message about "usb short detected, disabling this port" ,which allowed me to find it.
make sure the cpu fan spins to full speed (might not post if it doesnt or draws to much power)
plug in a mobo speaker to get any beep codes
i had a gigabyte board that would mostly not post, but would sometimes (and then run ok). i tracked the problem down to a faulty usb hub that the keyboard was plugged into (as an extension). once removed, it boots flawlessly every time.