You may not eat in the Underworld if you ever expect to leave again.
Dis Pater is an angry god. Well, not so much angry as really annoyed. Like many people in management, he's been promoted past the level of his competence—and Persephone knows it. It's always good to have a layer of ruthlessly competent middle management beneath you to keep you afloat, but you do not want said middle management to know how much you rely on them. Persephone knows; Persephone doesn't lick Dis Pater's boots, and that means she doesn't consult with him when situations arise. She handles problems with iron grace, and occasionally briefs her husband afterward.
And that's why he's such a fucking stickler for food. He can't control the Underworld, but he can control food. His own, and everyone else's. He tempts new arrivals with plates of figs and grapes, and then whips out the terms of service as soon as they swallow their first bite.
Persephone, unbeknownst to him, rolls her eyes whenever this happens. The food rule didn't even exist before her. The food rule was just a convenient way to arbitrate the dispute between Dis Pater and her mother. It's not like six pomegranate seeds really made the difference. He'd already proven a good lay. She'd already decided to stay.
If you undertake your travels to the Underworld while you are yet a living being, you may wish to sew your mouth closed—black thread is best. It is the surest way to avoid the temptations and escape with your soul intact. If you are deceased, you may disregard this rule.
Respect the guardians and the gatekeepers.
Nothing pisses off Persephone more than the fools who try to tempt or seduce her. There's payment for services not yet rendered, and then there's just foolish bribery. Have you heard the story of Theseus and Pirithous? They both pledged to marry daughters of Zeus. Guess who spent a good decade sitting in a Chair of Forgetfulness, chained there by ropes of snakes, until Hercules rescued him? That was Theseus, and he was the one who didn't try to steal away Persephone. Pirithous is still sitting there. And lot of people blame Dis Pater for that (he tempted the two intruders with a feast), but where do you think Dis gets all of his food? The Underworld is not known for agribusiness. Demeter has a hand in that, too. They're all working against you, and you have to know how to finesse them.
There are a thousand ways into the Underworld, and as such, a thousand gatekeepers. No way is clear, even if there appears to be no guardian. Do not assume, just because Bullfinch or Edith Hamilton did not categorize them all, that these gatekeepers do not exist. And their requirements differ widely. You might want to give ten drops of blood to the blind cavefish that guard the Acherusian Lake, but typically, multi-headed monsters like to eat things that bleed. Do not offer blood sacrifices to Cerberus, for example, or even his brother Orthrus (who has one fewer head than Cerberus and may thus appear more friendly); they both have appetites for live meat, and are excellent at keeping out the living and letting through the dead.
Pick your paths well. If you cannot re-read the relevant parts of The Odyssey, The Aeneid, and the various Homeric hymns, you should probably hire a guide.
Seek not to name the nameless.
Persephone and Dis Pater do not go around calling each other "Persephone" and "Dis Pater." Dis calls his wife Korē, which means "maiden"—she was one when he met her. Dis Pater (meaning "rich father") isn't even his name at all. Persephone calls him Hades, which means "unseen," and avoids the incestuous and absurdist meanings behind calling her husband, basically, "Sugar Daddy." "Hades" fits Dis Pater well, perhaps because he owns the Helm of Darkness and can tromp around invisibly all he likes?
The true name of the Lord of the Dead is unknown to all but himself. Perhaps he doesn't give it out because he can hear the name if it is spoken anywhere in the world, and that would be like getting poked in the head all the time. Or perhaps to know his name is to gain some power over death. Or perhaps humans knew the name once, and deliberately forgot it, because naming the nameless is the Big Forbidden.
Persephone's true name is also forbidden. It is hidden in the Underworld, which is a place of darkness and mystery, a land of loam and rot and gestation and blood and seeds and homunculi and spermatozoa. The Underworld is the rest point, and the secret name of Persephone is the spark that induces rebirth.
You do not enter the Underworld lightly or casually or to seek a wife (as Pirithous learned); you do not leave it lightly or casually or to punish a wife (as Sisyphus learned). (If you do anything at all with regard to wives in the Underworld, it is to recover lost ones.)
Crossings to and from the Underworld are not pleasure trips.
For the living, a crossing is a rescue operation, pure and simple. You go down to retrieve something you lost, something you cannot bear to be without. Someone.
For the dead, it is to return home.
In either state, living or dead, you do not call anyone by name. Just in case.
If you are granted the right to leave the Underworld, don't look back.
Persephone doesn't like hesitation. She pushes people to make quick decisions, because that's how she decides for herself. That's the one thing she's always liked about her husband: he dragged her down to Hades, and he didn't think twice about it. It wasn't the wisest choice in the world, kidnapping the daughter of Earth and Thunder, but at least he stuck to it, by gum, even when Demeter tried to plant his testicles like pumpkin seeds.
Looking back when you're moving forward? That deserves punishment. Persephone likes the style of the god who turns you into a pillar of salt for that sort of thing; she wishes she'd thought of it for Orpheus. But she was a little too fond of Eurydice to make her witness such a dreadful thing. And it wasn't Eurydice who looked back, now, was it? The proper party was tormented. Persephone hooked E. up with a sweet hero-sailor from the Argo in the meantime, and has it on good authority that Eurydice, for one, is not looking back. But maybe that's attributable to Lethe as much as anything.
If you lack the self-discipline to keep your head straight and your face pointed true on your return journey, consider a neck brace. If it is more that you doubt the deities will keep the bargain you struck, you are not going to fix anything by noticing the problem early. This is not a supermarket, and you can't just have some bag-boy run back for a price-check because you're diligent about watching your items get scanned. No: this is life or death shit, epic shit, and there are domains and powers and godheads and god-egos involved, and if someone with more godhead than you reneges on the deal, you will have to take it to arbitration sometime later. Just pretend you're Harriet Tubman—who knew a thing or two about conducting souls out of Hell: keep your eye fixed on the North Star.
Coming or going, you have to pay a price.
Whether it's a simple obolus left in your mouth by your relatives for Charon's ferrying fee, or a song of surpassing mortal beauty to charm a goddess, tolls must be paid all along the way to and from the Underworld. Commuters hate tolls—thus EZ Pass was invented—but even Persephone has to punch her timecard, and she's the Queen of the Sunless World.
Of course, the tolls for a queen and a wife, a goddess and a daughter, are not measured in dollars or obols. Her bill comes due with her mother's sobs, her husband's glower. There is always a reckoning. Whichever direction she travels, she loses a bit of the skin of her soul.
So when souls come around and don't have the fee, it's hard to be sympathetic anymore. In the early days, Persephone used to protest the treatment of the poor and the friendless. Dispassionate Dis Pater never answered her petitions, never acknowledged receiving them, beyond the first time, when he said only, "The price is the price."
Now, with her psyche slowly turning to scar tissue, she understands.
If you do not have any precious metals or gemstones on your person, and you are incapable of providing instantaneous artistic entertainments such as juggling or performing the moonwalk dance step—you may be asked to turn over your first born child, your hymen, your foreskin, or the memory of your mother's smile, depending on your pre- or post-deceased state. This is your choice, of course, to pay these fees; I would suggest to those without skills or extraneous body parts, do not enter the Underworld.