Its both system and the attitude. But i think we should start with the system. The authority (Government for a public place, Indian railway for the railway station, school/college authority for the campus...) should put more garbage bins and should frequently collect and dispose off.
As of now most people will find it weird if someone is carrying a carry-bag or searching for a garbage bin. But if these weirdos continue with their attitude and if the authority supports it, people will change.
For example, during my college days, I don't mind throwing a cookie wrapper in my campus while i carry them till the next garbage bin when I travel in south Mumbai (where Taj hotel and Whankhade stadium is). Why? you cannot find a single garbage bin in the campus and people litter everywhere. But in south Mumbai area (atleast the marine drive) you will find one every 500m or so.
So I guess it starts with the system.