You can only have so many pies in one day - and the ones at the 7s are rank at best - so if you're a big eater an early pit-stop at Maccers, KFC or any other fine fast food establishment is advisable. That said, although we're not normally fans of drinking regulations, we think lining the stomach for the 7s is cheating. Plus, you won't care what you're eating by midday. The most important factor on the Saturday, bar none, is positioning. Being in the South Stand's a given unless you're pregnant, and to be right in the thick of it you need to get their by 8am, latest.
A little gem here regarding the lavatory situation. You're going to need to pee, a lot, and the queues and general filth of the toilets is infamous. We once walked into a crowded W.C. to see a man in a Donkey Kong outfit crouching on the floor shouting, "Don't judge me! Don't judge me!" while he defecated on the floor. We're shooting ourselves in the foot by giving out this advice but you should go into the toilets through the 'exit' - you'll stroll straight in. Finally, don't be a wet fish. If someone throws a pint of piss at you, throw one back. Laughing. And don't make plans either. Roll down to Wan Chai and let the evening roll away.
Best of luck.