The most important part is not for force anything. I've learned from years of experience that you can't force a song that doesn't want to be created and you can't force a band to get behind a song that the band isn't enthused about at that particular time. I play several different instruments other than bass....this helps not only in the writing process (as far as coming up with the original inspiration), but it also helps to invision what the other parts in the song might be. This doesn't mean that I necessarily sit down and chart out what everyone else will play...it just gives me an indication of how things might work out. Personally, I used to write a lot on bass, but am finding that I've been writing a lot on acoustic guitar lately. I can't remember who said it (famous musician in a interview), but I remember watching a clip of someone on a tour bus say that if you can write it/play it on acoustic, then it will probably be a good song regardless of how the final instrumentation comes out.
It also helps to surround yourself with other talented people. The fact that I play in a band with an incredible drummer and two phenominal guitar players who also contribute ideas really takes the pressure off of me. I'm left to devise my own bass lines to every song we create (which usually just happens over time as a result of jamming and playing the songs over an over again...working things out and experimenting with new things in rehearsals), and I'm also only 1/3 of the songwriting equation. Sometimes someone will come in with a song that is virtually completed, and sometimes a song just develops from a feel, a riff, a chord pattern, or whatever. Again...the key is to not FORCE anything. More often than not, the process should just happen naturally. At least that's how it has worked for me.
