What is the most effective way for a VFR pilot to scan the instruments during manoeuvres?
A VFR pilot shouldn't really be "scanning" the instruments in the sense that an IFR pilot does; he should be scanning the sky for traffic (see my first answer) while using his peripheral vision to maintain attitude.  Yes, he should periodically (and briefly) check the instruments to make sure nothing is amiss, but then go back to scanning for traffic.  I wasn't taught any particular "scan" for that, just to look at whichever instruments seem most useful for what I'm doing at the moment.

If you're having trouble holding proper altitude and airspeed as a VFR student pilot (from a comment to other answer), you may be spending too much time looking at the instruments rather than not enough.  That may seem counter-intuitive, but the key to both is attitude.  Get the right attitude, using the artificial horizon or other instruments at first if necessary, then look outside at the real horizon (in particular, the relation of your nose to it) and then just maintain that picture while you do your traffic scan.

The only time a VFR pilot is supposed to be "scanning" the instruments is during hood training.  For that, I was taught the "T" scan: assuming you have a six-pack in the standard arrangement, you focus on the AI (top center) and then briefly look left at airspeed, then back, then down at heading, then back, then right to altitude, then back.  Repeat until the hood comes off.