Cecily Winifred is gorgeous, btw.
I think if I used
Cecily, I would just call her
Cecily, except every now and again maybe Cess, Cece, Silly, etc. like you mentioned. I don't care for Cess or Cessa on a regular basis--reminds me of a sess pool. :-/
Lily could work, but
Cecily is such a pretty, underused name, it'd be a shame for her to start going by
Lily most of the time. I guess one doesn't need to worry about that yet, though, huh? :b
A few other ideas (mostly just for occasional nn's):
Cecil - SESS-il or SEE-sil
Lee or
Lia or LeeLee
Essa or
EssieCilla