I've recently installed a new motherboard but now everytime i want to turn my PC on the screen just comes out blank and i have to switch it off with the power button. If I clear the CMOS first (by switching the jumper on the motherboard) and then turn it on, the PC works no problem. Is there something I should change in the setup to stop me having to do this everytime I start-up?

You should not have to clear CMOS every time you want to successfully start your system.
See if installing a new CMOS battery helps.
If no luck, then the MB may be defective.

Definitely get new CMOS battery. Chances are that's the problem.

If that doesn't take care of it you may have a loose component (ram, video card etc), or possibly a bad component causing problem during POST. Reseat all components. If that doesn't help you may need to do a bare bones boot to see if you can isolate a possible problem component.

It's also possible the PSU is under powered or bad.

System specs?