Making small gouges in the clay can help build up speed, as do sweeping arcs instead of sharp lines.
Stabbing actions at the screen when approaching the bullybeast work better than rubbing your pebble to gain speed.
Try to avoid anything with an '!' over it. It means it's too big for you to absorb at present and it'll cause you to lose precious clay.
The amount of clay you you managed to collect will be totted up and added to whatever you already have in the bank. Hit the Make button to see what you can make with the amount of clay you have.
To get more Power Plays, you'll need to cumulatively fill up a distance meter.