Honestly, I have trouble lasting even a day.  My basic strategy is to continually be looking for food.

-Go to the Al Fresca Plaza, stock up on food, leave when there's none left to pick up (hopefully with some left in my inventory), go to the Entrance Plaza, repeat, go to the Food Court, repeat x10, etc.

-Stay away from the freak in the meat room that throws the cows lol

-Once I run low on food throughout the mall, return to Al Fresca and make the rounds beating survivors/bosses.

However, I can never last more than a few in-game hours on that last step.  Help?

Make sure you get the books that make food more effective, then when you have those and a decent supply of food you wait around while your health slowly drains away. If you're going for the achievements I suggest you have a good book ready because it will take a lot of waiting.

Actually, I'm going for the lightsaber

And I completely forgot about the books. *facepalm*

Thanks for the tip.

1. You must have level 50.
2. Get a kitchen timer to ensure that you can leave the game sit for awhile, and not die (as your health constantly drops)
3. Get both magazines that help increase the effects of food

Preferably, you'll have the Mega Blaster unlocked (I can't remember what achievement gives you it, but it's very helpful to have it for the baddies).

Start off by getting both food magazines. I am unsure where they lie (I want to say in the easternmost and south-eastern most book stores), but obtain them ASAP while getting food items along the way.

Once you've obtained a few food items, as well as a gun or sword (a sword book helps too), find a good place to hide. Find out how long Frank can survive on the amount of food you have (usually 1 minute per bar...give or take), and set the timer for that amount minus one minute, to ensure you can feed in time.

Slowly use up your rations as need be. Save one food item, so when you go out 'hunting' again, you'll have extra just in case.

It's not hard, just time consuming. My profile should say I have the 5-day, but not 7-day achievement. I was mid way through day 6 when I died due to starvation - I set my clock for one minute too long.

I think it's Zombie Genocider.

@OP If you don't have that one get it first, the mega blaster makes everything a lot easier.

Always have a hard time with Genocider, too... I tend to just go on joy rides throughout the underground, but I always break down at the most inopportune moments...

Just make sure you switch cars periodically.  When I got it I started in the parking lot above ground and got in the sports car, drove it down underneath the supermarket and got in the truck, drove that over to wherever the white compact car is parked, then drove that back above ground to where the sprts car had respawned.  Try to avoid hitting too many guys with the gas cylinders, and stick to a repetitive route, then it shouldn't be too hard.

it makes me wonder how many zombies we will have to kill in dead rising 2 for the new zombie genocider or what every they call it.  it would be kinda cool if after you killed all the zombies none of them were left.... 54k down 6 billion to go 

There's a good guide for this at GameFAQ's, but I'll give you its basic rundown.

First I'd get Max Level and the Real Mega Buster. For Genocider plan out a good route in the tunnels and make mental notes of where you get another vehicle if yours break down. Once you've got an idea of the vehicle's general limit stick to a routine. Bring lots of food for when you get stranded and save every few thousand kills. Make sure after you get the Zombie Genocider Achievement you finish and get one of the proper endings to unlock the Mega Buster. Friend of mine killed himself to start a new game and didn't get the Mega Buster because of that. =P

As for survival mode, there's three books you'll want to grab. Health 1, Health 2, and Survival. They give you +50%, +50%, and +100% to food effects. There at Sir Book-a-lot at Wonderland Plaza, The Sinster Read at the Entrance Plaza, and an empty shop next to Crislip's Home Saloon, which I think is North Plaza. Once you've got those and the Real Mega Buster head for the Park and kill the three snipers. They all drop good stuff, grab what you can and use a zombie to jump on the roof of the small resting spot.

After that it's mostly about how do you want to spend the 15+ hours. With those books and the food from the snipers, there's enough remaining static food, and enough places where you're untouchable that you can run out the clock. (Use the guide I posted to see exactly where.) Grab every food item in an area, get to a high spot, burn time untill you're almost out of food, move to the next area. The guide I posted suggested playing something on your DS or PSP. My TV has PIP, so I watched movies making sure to watch my health bar as it ran down. The same guide also mentions the wireless 360 controller will shut off automatically after 10 minutes and pauses the game. I think you can live for about 12 minutes at full health, so that can work as a timer.

If you get tired yourself, pause the game and get some rest. Just make sure nobody is going to mess with your 360 while you're gone and you can come back to it later.

EDIT: If all you want is the lightsaber, you only need to make it 5 days. All though if you get that far, you might as well got for the whole 7 anyways.