I recently was lectured on this ... lemme see if I can remember what was said ...

If you're using proper posture so that your upper body is being supported by your core, not your hands, you'll be more likely to use your glutes.

Try thinking of pedalling as pushing/pulling forward and back, rather than up and down.

One suggestion was to try pedalling in a much higher gear than you normally would, just to get a feel for proper technique. When you have to pedal hard, you'll find your body more naturally using more efficient technique (or so I was told).