A lot depends on the song. Lordathestrings is right to say not every song needs a solo, and Martin's idea of bringing new techniques to an existing solo is a good one. Some songs I play the solo note-for-note the same every time, but others I have a starting phrase and an ending phrase, and then just free-fall improvise in-between (that's where the joy is!). The ending phrase is usually important to the other guys in the band as a cue that the solo is finished, and it's time to pick up the song again.