rules of thumb for cross os code

[DwBear75]

I am considering using python as a replacement for a lot of bash
scripting that I have been doing. I would like to be as cross platform
as possible, writing scripts for both windows and linux. Are there any
guides are general rules of thumb on
1) keeping code os independant
2) nifty lambda's or other things to create functions fit for the
environment
3) capturing return codes of other executables called from python; ie
use os.system, or popen? or ?