I found hair clips to be good for adding volume to my hair.

Clip-on hair rollers are great for adding volume to your hair roots. They're also convenient to use because they're clip-on. I bought them at Olive Young, but these days they're sold at Daiso too. It turns out Daiso is actually cheaper, haha. The weather has been humid lately, so the volume at my roots keeps flatting; you should give them a try!