Do's:

Flush
Flush again,
Flush some more
Reverse flush,
Finish up with a flush,
Get all flushing agent out of system.
Coat o-rings with Nylog, if that's not available coat with refr oil
Put in the correct amount of refr oil
Connect up your filter drier last thing before evacuation
Evacuate thoroughly
Make sure you have the correct seals for the R4, there are different ones
Use mineral oil and R12

Don'ts:

Don't even think about a new compressor without thoroughly flushing
Don't use anything but R12 and mineral oil

Good luck,