🚀 go-pugleaf

I have just signed on for a research project at school, and I have been given
a handful of handwired circuits that need to be redone on etched p.c. boards. 
(A hazing ritual??)   I have done a few boards in my time, but I need some
advice.   The crucial board is a voltage probe with variable gain, which feeds
a 12-bit ADC.   It will not see high frequencies, but as the voltages are in
the microvolt range, noise will be a problem.  How far apart should signal
lines be from power lines?  How much does adding a ground plane on the front
side affect noise levels?  Etc, etc.   

I would greatly appreciate any advice on personal layout strategies
or a citation of references.   The p.c. board articles in the hobbiest
mags I have read did not discuss these layout questions.

     --Larry Liska