I too have thin hair. It used to be really long, but I recently cut it shorter (just below my chin). I aim for the modern/toussled look as well. Let me tell you, it's been no easy chore figuring out how to get to fluff, but I finally figured it out.

There is actually a few different ways to get the desired look. I wash my hair with Pantene Pro V "Full and Thick" shampoo and also use the conditioner. After I towel dry my hair, I use a product which adds volume. There are many out there to choose from. I would just experiment. I use some from a salon called Kenneth's. I then blow dry my hair upwards to give it some lift at the roots. Never mind the mess it looks during this. Once it's dry and fluffy, I use a flat iron to straighten out and/or flip ends when I want. The key with the flat iron is to only use it on the lower half of the hair shaft. If you go too high, you loose your lift. Then I use a small amount of either styling putty or a styling pomade in the areas I want to look toussled by spreading it evenly on my hands and fluffing my hair. Then I strategically place certain hairs and use hairspray to finish.

Hope that helps!