I have a chronic problem with my digestive system that is definitely affected by how much fiber I eat... I'll spare you the details.

I have learned how to judge when I need more or when I'm getting enough fiber, but I couldn't give you an exact number of how much I need per day.
The easiest way for me to increase it is to use that clear, dissolving fiber powder when I cook. I stir it into anything, and can't taste it at all. It's supposed to work in cold beverages, too, but I haven't tried that.
I know the experts tell you it's best to get it from fruits and vegetables, and I'm sure they're right, but when you don't get enough that way the powdered supplement is great.