Ahhh, the good ol' major 7th chord, your ticket to making millions of people depressed, or if used differently completely uplifted. The major 7th is a magic chord, and when doing your magical bassline, it helps to put emphasis on the major 7th of that chord (in the case of C major 7th, the major 7th is B) , as well as sticking to your standard pentatonic scale and using notes contained within the tonic triad wherever possible. Have no idea what I'm on about? then look at this example. This is a standard I IV V progression blah blah, but the chords have added 7ths and 6ths for that extra magic touch. The key is easy peasy C, and the rhythm is pretty simple. To get that ultra slick pink floyd esque sound, slide between all the notes that are on the same string, use hammer-ons and pull-offs if needed, and add lots of chorus and reverb to get that sound dreamy and creamy (mmmm!). If you like this lesson feel free to make it obvious, likewise if you hate it, cos y'know, idiots like me need to be told. I like to call this piece "bath time music". I have no idea why.