What can I use to drive the PCs from petty theft to something more interesting from a plot point of view?
Drive is another word for motivation, my friend, and like a few television commercials say "What motivates you?"
I am going to attempt to lay out some basic motivators that correspond to the Human Psyche with that slight fantasy twist as evidence.
You cannot forget your basic motivations for basic existence: Food and Security.
While food may not be that exciting on its own, consider food becoming scarce, requiring bigger heists or longer hours to afford the same meager meals.
- How do you motivate them? An empty belly and a bare fridge.
Security is a great concern for those that like to sleep the whole night. A good night's sleep interrupted by vagabonds, murderers, other thieves, demons, monsters, and what have you is something you can only stand for so long. The cost of Security is also never shallow. Guards to fend of murderers and monsters, sigils to fend of demons and other creatures of the night all cost money.
- How do you motivate them?: Tortured, if broken sleep.
If you take a step up from the basics: Money & Sex.
Money. (... yeah, I think everyone's basically covered this bad boy. Next!)
- How do you motivate them? Broke until employed.
Sex is a motivator for every young red-blooded boy ages high-school to withered old man. Sex motivates women just like it motivates men, but occasionally in much more sublte ways. Love, or the attraction to a particular member of the opposite sex (or same gender) has garnered box office hits, written books, sailed ships, etc, etc, etc. The more broad appeal to lust isn't lost on anyone who's experienced any media featuring "ladies of ill-repute" in red-light districts and the like. You can definitely get people to pay for the service rendered by these ladies of the night, and their services aren't cheap. Though sometimes, you don't always pay with money.
Sexy isn't the same for everyone, or for both genders, for that matter. For Male GMs with Female Players, remember that "Sexy" isn't the same for them as it is for you. Sometimes sexy is funny and caring, or mysterious and dangerous. Treat "sexy" as the word "attractive" and you may have an easier time of motivating female players with "sexy" npcs. After all, the quiet, brooding, motorcycle-riding, witty loner with a mean right hook doesn't have to be as big as some guys to get the same sort of reaction. The key words that you may have to switch around to motivate female players are "sexy", "attractive", and "successful". (Attractive men are successful. Sucessful men are sexy. Attractive men are Sexy.)
On the more personal side you can reward players with "Sex" by making them more attractive. Human beings are the veritable epitome of maximizing sex appeal. We have had makeup for eons, powder rooms for centuries, mascara that elongates your sensual lashes for decades, and Axe Body Spray(tm) for years. A bottle of "Spanish Fly" and the promises it brings is definitely a reward based solely around sex. Getting items that enhance sex appeal is most definitely a reward well understood and eagerly received. Especially when we spend anywhere from $50.00 to 1 arm and/or leg in real life to acquire such goods.
(To clarify: I'm not saying have graphic scenes in your game, but enough under the rug and understood exchanges should be more than enough to satisfy this.)
- How do you motivate them?: Hot babes, ... for a price.
- How do you motivate them?: Successful Men that are available if you could just get them to notice you...*
And before we climb any further up this ladder of Motivations, let's take a break for a second and think about rewards and punishments, ie, "The Boons" and "The Banes".
Nobody likes getting punished, -- (taking away stuff, getting bad stuff)
unless they think it's a reward. -- (getting bad stuff to hurt enemies)
People like getting rewarded, --(giving good stuff, taking away bad stuff)
unless it feels like a punishment. -- (taking away bad stuff that helped you before)
ex: You can gain 5 mercs by joining the Don (and doing what you were going to do anyways) OR you could get a kick in the teeth by 5 goons for encroaching on the Don's territory.
IF those goons drop sweet loot and don't kill anyone, it's probably really a reward.
IF those goons seriously wound the innkeeper and you get kicked out onto the snowy streets, it's probably really a punishment.
Remember to give rewards occasionally for doing the "right" thing. Tying back to the sex motivation, that really hot barmaid could totally dig bad boys. -- Reward players for joning the group and make sure they understand it comes from joining That group. String rewards together to form a chain of motivations.
Carrot and stick my friend. Show them the hot babe. They want the hot babe? Go this way. (Man is she hot...) Occasionally, give them the carrot. (Her name is "Angelique") Not ALL the carrots, just the one. -- Oh look, another, bigger, juicier carrot! (Her name is "Nautica")
Another rung up the ladder...
Allies can be incredibly useful, especially when you don't want to do things yourself. After all, why bump off the local thieves guild, when you can ally yourselves with them and get a cut for helping them out with problems. Forming allies that grant you goods and services is what people do on a daily basis (Ever play on your friend's Xbox?). This is what nations do (Trade treaties). This is what your players can do.
- How do you motivate them?: Show them who has what they want, how easy it is to be their friend.
Authority is more ephemeral than money, but can give the same results. You could pay a fleet of mercenaries to ransack the town OR you could command a platoon to commandeer a town to stage a further assault and re-supply you troops. The power of doing so isn't lost on anyone facing a powerful enemy -- After all, that's why you team up in the first place: more hands make for light work, especially if that work is slaying a dragon or taking down a kingpin. The power that authority wields can be felt in the number of people at your command and or the power those sub-units command.
Even if this authority is authority over the self, after being ensnared in a command structure this is pretty compelling.
- How do you motivate them?: Show them how big their authority could be.
Longevity is a bit more esoterically than the other motivations. It usually comes towards the later stages of life. In human beings, according to Erik Erickson, after you've had the chance to make it in the world, form your own identity, and find love it is time for you to make your mark and leave a history behind you. This stage of life is identified by the amount of Care you leave behind for your progeny. Do you leave behind an empire of gold, or a swamp and a salted field of ashes and despair? This is the late life crisis, and it motivates very well once you've reached that stage. In reality it is the source of commercials for life insurance, worrying about "who's going to take care of my loved ones once I'm gone?", and nagging your descendents for even more grandchildren.
- How do you motivate them?: Tell them how little will be left when they're gone.
final Boss last rung.
Self Actualization, according to Maslow's hierarchy of needs, is the pinnacle of achievements. Back to game mechanics terms, it's gaining the final level and maxing out your stats. In terms of plot, it's your great destiny -- wielding that +infinity vorpal sword and cutting off the head of death itself.
- How do you motivate them?: Show them what it's like when someone achieves their destiny, and how epic it really is.
What can I use to drive the PCs from petty theft to something more interesting from a plot point a view?
Food, Security, Money, Sex, Allies, Authority, Longevity, Self-Actualization, Pain, and Pleasure
Promise with it, dangle it in front of them, give it to them when they do 'good', take it from them when they do 'bad'. Spin it however you want, but it should stem from those eight things.
(As ALWAYS, make sure your players are okay with it before you start stringing them along. If they aren't having fun when you do it like this, do something else! If it's NOT FUN, it's NOT WORTH IT.)
Sources: General knowledge of Psychology (degree), Wikipedia (as linked).