A good bassline is something i think many people overlook in a song. A bassline is, however, essential in letting the listener put certain chords into context. One song can sound completely different with different bass notes, or with a different style of bassline (as far as rhythm, instrument, technique). Over the years i've learned a few techniques for making decent basslines in my compositions. Some of these tips apply to all genres, though in particular, they might be especially helpful for those who write rock, pop, funk, jazz, video game, ska, etc, anything with a good beat, and maybe even certain styles of techno.

Generally, a bassline can't be too complex while a melody (lead) is going on. It's a bit taxing on the listener, i'd say. If there's a nice, active melody and an active, (relatively) complex bassline, the two clash, the listener has a hard time focusing on one thing and the music begins to suck. For the same reason, two people can't improvise solos at the same time in jazz. Stuff clashes.

To start writing a bassline, make something simple for one or two measures (or however long your first chord in the chord progression lasts). What i'd do next is pretty much copy the same rhythm and the same pattern the notes take, but with the next chord. Do this for all chords. After all of this is done, you can make a few imbellishments. Maybe add a few grace notes, or a few variations from the maeo rhythmic scheme.

For something a bit more advanced, try synchronising bits of the bassline with the drums. A bass note sounds a lot fuller and supportive if it lands with a bassdrum. It kicks more and feels well mixed, and tight. on the whole, if a bassline interracts well with the drumline, it proves even less taxing on the listener, yet more enjoyable because the groove is being held by both the drums and the bass. At other times, though, it can be good to establish a solid, repedative rhythm, and use the bassline as another rhythmic source, syncopating, going against the drumline.

When a chorus comes, it's often nice to dummy the bassline down a whole lot. Maybe just make it the bass note of the chord, repeated, with a simple rhythmic pattern. Sometimes though, if the lead in the chorus is made a bit simpler, a little bit more room for the bassline is allowed. Most often, the chorus is the part of the song most remembered. Often times, everything in the song gets simpler in a chorus, rhythms and phrases get repeated a lot more, and lines become a lot less complex.

These are just a few simple tips... I've seen friends begin composing and they tend to go all out with everything. Insane, complex drumlines with intricate basslines and intense, crazy leads... Maybe try some of these tricks out, or tell your bassist a few of them. Sometimes less is more.