Fiber supplements like Benefiber are ok, but they do have sugar in them so I wouldn't go crazy with them. The supplement I have has 45 calories for every 3g of fiber.
If you're having a hard time calculating fiber content, read the label if there is one--it'll tell you. If there's no label--such as fruits and veggies, fitday.com is great for that. It tells you the fiber content in the nutrition list.
For homemade fruit and veggie smoothies, just log the amount of each fruit/veggie juiced into fitday or some similar calculator. That'll tell you the fiber content. I would guess it's pretty high.