The above flash program is a tool I use for playing around with bass lines.
There are zillions of books on the subject of building bass lines. I know because I've bought a fair number of them. I'm sure the books are just great. I wouldn't know. I'm too much of a noob on my bass to get much value from these books.I've poked and proded and tried different things to penetrate the world of base lines. I'm sure every bit of experience has contributed to my gradual growth on the bass. If nothing else, my fingers are getting stronger.
Strangely enough, the thing that has been the most inspirational to me was my hand drumming books. In an effort to learn Djembe and various world rhythms, I've collected some fun books on the topic of drumming.
Many of these books have diagrams with boxes filled with "stars", triangles, or squares to indicate which type of drum hit to use at time mark.
One day it occured to me to replace the indicated drum sounds with notes on the bass. (To keep things simple I worked with minor pentatonic scale. You could do this with any scale or triad.)
So following the drumming diagrams, I substituted bass notes for the different drum hits. I would use the root for "star", 2 for the triangle, 3 for circle, etc. This worked like a champ. It was a major door opener for me. Of course, your mileage my vary.
If you are intrigued by the idea, Get yourself a copy of an inexpensive drumming book (such as Alan Dworsky's HipGrooves) and try this out.
By the way, in addition to getting you started on bass lines in general, using this method you might even break free from the lame world of 4/4.