This is a little too broad questions.
-What kind of technology this PCB use, pure digital, Mixed Signal, Analog?
-What is your concern, noise, emi, etc?
In general.
-Avoid mixing analog ground with the digital ground, to avoid noise from digital getting into analog circuit. Tie them only at one point. It will be better for EMI to tie both ground in a very big piece as possible but make sure you already seperate aganlog and digital ground.
-For low layer board, pay a lot of attentention to maximize the ground area. Put ground tie vias to get some ground into the area.
-Use thick trace to connect components to ground.