Well the question is what do you mean "a good layout".
I.E. if we talk about baseband layout design then almost throughing devices is a good layout ( not exactly ) but you get the meaning).
A good practice is to go above the minimum of technology ~20%. I.e if you have M1 with min width=0.28u then in order to avoid malfunction in real world it is good to use 0.4u and keep this in all layouts as minimum.
Another generic good practice is to use more than two contacts from layer to layer, using also the technology recomndation of sourounding metal pads.
But is this topic a sum up of generic terms for layout or is there a specific request?
to better benefit on how to have a good layout...first you need lots of experience in terms of reading materials in good layout practice..and more hands-on self-paced or at work experience...i would recommend reading IC Layout basic and IC Layout essentila by saint & saint.
try to place them as near as possible so that it will require minimum wiring. as we know that metals have resistance and parasitic capacitance with the substrate or with any other layers. long wirings will increase time delay equal to RC.
Before starting any layout, make sure you come up with a good placement of the devices.Take into consideration routing according to the functionality of your design.Proper placement will save a lot of time
PLI is for Photo Lithographically Invariant, and this type of layout is better than mirrored or stepped ones, because it looks the same no matter how you turn them. This has a limitation for the very small devices, where you must use stepping.
try to minimize your metal layer. because your contact create resistance and noise. Try to locate all your cell smartly and do the floor plan before start your layout,,
Its very important.