Here's what I know about influencing prep so far, gleaned from numerous forum threads, reading HELP files, a bit of personal experience, and actual conversations. It's a given that Influence, Discipline and Dramatics are the three main skills needed (or highly desired, in the case of the latter two), and that Charisma is the primary stat.

Why am I posting an overview? Well, because I've never seen all of this information gathered together in one place. Eventually, I might put an actual guide together that covers everything in detail.

Charisma Buffs
BardicPresence (+2), Throne (+2), Penumbra (+2), Netzach (+1), Karmic Beauty Blessing (+2), Minor Beauty Domoth Blessing (+2), Herofete (+1), Astrology (various), Populus (+1)/Fortuna (random +1) in Rituals, Divine Favor (+2) etc.

Maximum is +8 weighted, +5 Charisma total. Only Titan, Demigod or racial stat changes bequeath permanent, non-weighted increases (?).

Perfumes
Jasmine = Paranoia, Musk = Seduction, Dragonsblood = Empower, Sandalwood = Charity, Vanilla = Weakening

General Equipment
Bromide potions, mana potions, Beauty enchant, Perfection enchant, Healing Scroll, Sparkleberry, Crow Cloak (influence undead), herb w/ Major Nature Blessing, etc.

Influence Beasts
BEAST TRAIN INFLUENCE -----, at Master +33% Beastmastery; ponies for Begging; teach them Cure Ego (for you) and Regenerate Ego (for them).

Influence Rune Artifact
Reduce attempts needed to succeed at influencing your average denizen by ~1; there is one rune per ego battle type.

Dramatics Use
PERFORMANCE ON/OFF, PERFORM -----/END; match performance to proper Influence type.

Influence Methodology
Don't use the same Influence ability repeatedly. Alternate between two or three within the same Ego Battle type.

Ego Battle Weaknesses
Friendly: (weak to) Charity, (immune to) Paranoia
Greedy: (w) Weaken, (i) Charity
Brave: (w) Empower, (i) Weaken
Sensuous: (w) Seduction, (i) Empower
Intimidating: (w) Paranoia, (i) Seduction

Willpower Conservation/Restoration
High Discipline, TOUCH BOARD in Glom Library, BATHE FONT in Celest, toggle CharismaticAura on and off adroitly, die and Conglutinate, rocking chair, manse Willpower regen artifact, campfires, Great Rune of Iron Will, etc.

Charity Clothing
Have a tailor mend clothing items a bunch of times, creating "rags"; Scarecrow Hat dingbat artifact.

Clothing Arsenal
Ten rings, two bracelets, a choker, a necklace, one anklet, a crown, a brooch or three, as much fine clothing as you can fit on yourself, and whatever else I've (no doubt) missed.





If you wish to correct any of the above or add something more to it, please feel completely free.

Okay, here's my main question: How can one know what caliber of prestige a given article of clothing or jewelry confers? Is it based on days of usefulness remaining, the commodities used to create it, the price for which a shop sold it, the Tailoring skill of the creator, its type (gown vs. dress, for example), or even the sumptuousness of its flavor text? A combination of two, three or more of those perhaps? Or will any unmended piece of clothing or jewelry do?

Piggyback question: Is there a way to determine what Influence/Ego Battle type a given denizen is weak to, other than trial and error or asking someone else?

Analyse them. Each personality is weak to one personality, and immune to one.

Wow, that's embarrassing... I guess I've been reading so many skill lists lately, I just missed it.

Taken from the Wiki:

Disposition Easily influenced with Immune to
Friendly Charity Paranoia
Greedy Weaken Charity
Brave Empower Weaken
Sensuous Seduction Empower
Intimidating Paranoia Seduction

With the caveat that Magnagora's guards are intimidating but are immune to both Paranoia and Seduction.

Karmic beauty blessing is +2 (weighted). There's also minor beauty domoth blessing (+2 weighted), herofete (+1 weighted).

Well, it makes sense that they're immune to seduction (which I believe makes them attack the influencers enemies?), and it could be mobprog-related that they're immune to Paranoia. It likely also extends to all guards, although in all honesty I've never influenced guards with anything but Empower.

Okay, apparently the prestige of clothing is determined by the people who approve a design, based on its flavor text and the type and quantity of comms that go along with it, generally speaking of course.

I just learned this from a city-less guy in-game who never influences.

There are also a number of abilities in various skillsets which also help to one degree or another. Harmony Anguti boosts influencing and debating. Stag Lightning increases equilibrium speed, and thus influence speed, as well as helping to avoid rebuttals from denizens. Crow Cloak or Necromancy Mastery to be able to influence undead. The list goes on.

Equilibrium boosts do not affect influencing, only Charisma.

If you recover equilibrium faster, you can influence faster

Except you can't, unless the help scrolls are all wrong, since equilibrium boosts don't affect recovery from influencing.

There are the Astrology buffs, too. Lion can add to your charisma, depending on the alignment of the stars. Astrologers also get a charisma bonus whenever Sun transits their organization's sign (Spider - Glomdoring, for example).

It's entirely possible that's changed, or I'm just misremembering. I had a mugwump alt which seemed much more capable at influencing than my merian, despite the poorer charisma, and I attributed it to the faster equilibrium.

You were around back when that was true, but it hasn't been true for, oh, four or five years now.

The caveat is that all guards are immune to anything but empowerment and weakening, actually. It's just no-one bothers with anything but empowering them, as all but Magnagora's are weak to empower anyway.

Also:
Populus in Rituals, +1 cha buff;
Fortuna in Rituals, +1 buff to a random stat (and thus may hit cha);
Laetitia in Rituals, heals ego (takes eq, thus only really used when you want to help -someone else- influence);
Influence in *Mancy/Druidry, influence buff while in meld
Undead-ness allows influencing of undead, meaning that a liched (not just lichseeded) person or someone under the effect of UndeadBlues (the Necroscream effect) can also influence undead

A few other additions to your list:
Beasts: Training a beast in being able to Cure Ego is very helpful (be sure to get Regenerate Ego too or the beast will run out almost instantly), particularly if you don't need to sip that often already.
Bards: Some bard songs can help with influence power or ego regen, though most are not really enough use to use unless you're the bard (and not even always then).
Willpower: The Great Rune of Iron Will outstrips all the other artifact methods of recovering willpower if you can afford it. Also, rocking chairs stack with campfires and manse gems, and odds are there's a manse somewhere that you can use that provides the rocking chair and gem for you.

A bit of general advice: don't be afraid to step out of the room when your ego gets dangerously low while influencing. People often worry that they're going to make it impossible to win, but if your ego is low, do it anyway. If you get low enough that you need to step out to avoid losing, and it's not because of bad luck, you're not going to win anyway. And if it is because of bad luck (like a run of lag), the "willfulness" that builds up during a few seconds recovering in the next room is so negligible, I'm not even sure it's really happening.

Totems' Sun helps with ego regeneration during the day, too.

Is there an upper limit to charisma, or a point where stat boosts don't have any further effect?

You can only get +5 (which is +8 weighted) of any stat, which would put the (non-DF) limit at... 24? I think that's what a Demigod Shadowsinger Faeling with +5 would reach.