Perhaps most helpful, although not strictly speaking required, is personal tragedy in the not too recent past. If your spouse, or child (a pet will not do) has recently died, you'll have to wait at least two years to apply. As a natural result of this tragedy, your house will have become a cluttered shrine to the deceased: their room will not have been modified substantially, random household items associated with the deceased will be considered sacred, even if the dearly departed would surely have tossed them the next day, had they not instead crossed over to the Great Unknown.
More likely, someone has shared your space with you, and moved on. Ideally, they will have left a lot of household furnishings, clothing, toys and so forth behind. This could happen as the result of divorce, children growing up, extended family members who were once down on their luck moving on, a roommate who you had one too many fights over the contents of the fridge with, etc. It's hard enough to keep up with one's own belongings; when the owner isn't you and isn't present and probably presents difficult unresolved feelings whenever contemplated -- well, you get the idea.
In a similar vein, when households merge, clutter is inevitable. Whether the result of two people sharing rent, shacking up, getting married or an adult child moving back home, attempting to wedge two (or more) households worth of living room, family room, dining room furnishings, not to mention unused exercise equipment, guarantees household mayhem and foolishness.
You may still be able to get on a Reality TV show without major changes in the occupants of your home, however you may have to sustain clutter over a longer period of time in order to generate enough of it. Accumulators (aka packrats) may acquire a relatively typical amount of consumer goods over the course of a few decades, but can become worthy of a Home Makeover by the simple tactic of never getting rid of anything. Ideally, this would be done comprehensively. For example, a true Accumulator would have the ticket stubs for every movie, play, or sports game they have ever attended. Every children's toy, however broken, every puzzle even if half the pieces are missing (or just the box remains), the packaging from every small appliance ever purchased, etc.
Once on the show, you have a duty to engage in a sustained struggle to hang on to your stuff. Otherwise, the audience will be suspicious that you just wrecked an otherwise ordinary house in order to get a truckload of new consumer goods. You have to show that you know exactly how to cross the room with no path through the crap, or we'll think you just spread it around the day before. You have to refuse to part with your collection of crappy snow globes, so the host will agree to Gift you with something worth thousands of dollars. Please cry, at least once. And when they send you to the hotel, look really stressed out. When you come back from the hotel, before the big reveal, we expect you to have had trouble sleeping, because you were so worried that the designer's hotel-like style (with a thin veneer as a handwave to your "style") will somehow be worse than the crap that you lived in.
Finally, to truly satisfy your end of the bargain, you should try not to have your home foreclosed on in the ensuing months and years. Ideally, you'd keep up appearances (showing that people CAN be reformed by truckloads of new consumer goods). Try not to be too sad in the follow up show a year or so later about having had to give up your collection of snow globes.
One last word: while it is tempting to think that a house cluttered by a great number of children and/or pets will get you on the show, the cold hard truth is that this hits way too close to the bone for most of the audience. You'll be fighting a lot of other people for this extremely rare representation of what we all have to live with. Every day. Without truckloads of new consumer goods, much less a designer prepared to brutally edit down our clutter and stack books on our shelves horizontally in aesthetically appealing ways.