Say I wanted to stockpile food or drive across the country. I may even have a legitimate reason to do so beyond mere acquisition or immediate self-interest -- to financially support or care for a relative, say, or move to a new job or business opportunity. How strict was rationing in practice? Was it easy to trade or bribe for extra coupons/stamps or an exemption? Were some merchants or ration board workers known to look the other way or fudge their records?
Aside from the legal restrictions on the things you mentioned, there was also a very large propaganda campaign encouraging people to donate goods, volunteer for scrap drives, factory work, and demonized people who weren't doing their fair share. Mediums like, pamphlets, posters, and advertisements were all utilized. There were also daily meal-planning guides published by the govt. for people to make the most of their rations Slogans like, "Do with less so they [Soldiers] will have enough." became popular.
That being said, there was still a black market. In the US, the black market primarily dealt in meat, sugar, and gasoline. All of which were rationed greatly. It wasn't extremely hard if you were able to pay significantly higher prices. It was still risky of course, and certain to make you looked down upon by many people.