The first one is the Aveda volumizing tonic. I did a full review on this last fall, but basically, I use spray this onto wet hair, and it just gives your hair really natural volume. It doesn't leave your hair crunchy, and to be honest, I don't really know how it works, all I know is that I'm running low and am definitely buying a second bottle (this one lasted about nine months, using it 2-3 times a week). If you are looking for an easy way to add volume, this would be a great place to start.
Now if you want to up the ante a bit, a texturzing spray is the way to go. I've tried several, but keep going back to the Oribe Dry Texturizing spray. I use this two ways - I first spray just a bit on the ends of my hair before curling - I've found that this really helps my hair hold the curl a bit more. Then once I'm all done, I'll spray a bit more at the crown for volume. Because it is so expensive, I try to be conservative with my use of it, but this goes such a long way. If you aren't ready to commit to a texturizing spray, simply using dry shampoo, even the day you wash you hair, will add enough product to your hair to give it some oomph. What did people ever do before dry shampoo?!
The last tier of volume producing products is the Aveda hair potion. I mentioned this in my April Favorites video, but basically, this is a powder you sprinkle into your hair and it creates almost a stickiness that thickens up your hair. Unlike the other two products, this one does make your hair feel like you have product in it, so I don't use it every day because I really try to minimize how often I wash my hair. I'll use this if I'm putting my hair half up (so I have more volume at the crown that also stays there) or if I have a special occasion.
And that's it! How to trick people into thinking you have thick voluminous hair when you really don't ;)