Although few pilots believed it at first, over the last twenty years it has become obvious that the Russian built RPG anti-tank rocket launcher is a capable anti-helicopter weapon. Fired in volleys (six or more), RPGs stand a good chance of nailing a low flying, slow moving chopper. Naturally, pilots have developed ways to avoid this sort of thing;

Do not fly low along streets, roads, canyons or river lines for a long time. 
Don't always take off and land from the same direction. This is often done because of weather conditions or the lay of the land (fewer obstacles on one direction.) Don't do it, for the bad guys can mass their RPGs along your usual flight path.
If possible, drop a fuel air bomb on a new LZ (landing zone) before landing the first time. 
Never fly the same pattern or formation while on patrol. Again, this makes it easier for an ambush to be set up. 
Never use predictable patterns of operations (by time, formation or sequence of events.)
Send in pathfinders (scouts on the ground) on any LZ before sending in the full landing force. 
When two or more helicopter flying together, always keep 500 meters between helicopter. This allows each chopper enough room to use their weapons against RPG gunners.