@Arkandel
Make it Easy - How?
For me? I think making it easy to do involves giving people the appropriate resources to do the job effectively: have all of your guidelines posted and there for people to follow. Not just 'here's the levels of involvement that require a staff approval' but give guidelines for, say, how much XP to give to antagonists; general guidelines for what antagonists should be used; assistance from staff directly when someone asks for help; pointers to past plots that might intersect with their idea.
Give Opportunities - How?
The big one I think is to let players play off of staff plotlines or the aforementioned big list o' staff plots. If staff run something, make sure the outcome is clearly posted so that people can play off of that in small or large ways. Give leeway to play out things, or at least ask for their base idea so you can give constructive criticism. I'm very much a 'yes, and...' or a 'yes, but...' kind of Storyteller when it comes to this type of thing.
An example from my Transformers days. On Transformers: Genesis they ran a plot where an Alternate Universe Rodimus ended up on the game's Earth. I played a medic and the staff had part of the plot be 'the Matrix gets a crack in the crystal and the energy is driving Rodimus insane'. So I asked staff 'Hey, I'm a medic, if I wanted to repair this, does this sound reasonable? <list of stuff>' The response? 'Yeah, go to town, here's some info you can put into the plot to seed further stuff'.
The outcome? I organized people to go beatdown Rod to get his Matrix, then interfaced with it, then found a crystal with the right resonance/frequency (which led to a nice space excavation and Decepticons trying to steal the crystal event), then transferred the energy and fixed it. The plot continued from there with info that I was able to seed and give further back to the game from the plot.
But staff being open and giving the opportunity to do stuff with it, was a godsend to keep the plot running even without staff directly doing something with it.
Be open to ideas - how?
Be a 'yes, and...' or a 'yes, but...' storyteller is a good way to do so. Not that you aren't, as I've never played under you that I know of, but being open to ideas (to me) means letting a player give you their full input on their idea, however baseline it may be, and then be willing to help them refine the idea.
Another example from my Transformer days: I played a sonic weapons specialist on the first Beast Wars game. One episode involved the Maximals getting stuck in animal brain mode after having to be in beast mode for a long time. I presented to staff 'Hey, I think this would be fun; how about a sonic weapon, stuck in the Maximals ship but hard to find, causing them to be unable to transform due to energon issues? Thus giving OC Maximals a taste of this, and Preds an in-game advantage' Staff helped me refine the idea and it led to a long term arrangement of plots (build the devices, test the device on Preds, attack on Maximals to insert the device, as the player who wrote the plot working with staff and Maximal PCs on finding the object, the final fight with another Maximal to get the frequency codes to turn off the device after they failed to find it).
Or a more recent anecdote: we have a player playing a dhampir in the LARP, who is using Merits and some powers to mimic being a Neonate Kindred in Vampire society. His idea? 'I want to play an Obertus experiment on the run from the Sabbat'. Staff's response after going over the viability of the idea? 'Cool. And, here's our thoughts...' and he took it and built from there for the character, and we're working with his backstory for plotlines to coincide with that.
Enforce staff - how?
I think 'enforce' isn't the right word I should've used there. I guess 'encourage and cultivate' the culture for staff to run plots that tie into the metaplot by giving them what you've given the playerbase at large for other plots: guidelines, timeframes, the theme head's goals for the metaplot and a stable of ideas.
And example from my history of M3. We ran a plot as staff where the Stardroids (space alien robots) judged the Earth to be unfit to continue to exist, and the timeframe was loosely set, and plot stuff was presented to staff that 'the Stardroids can be challenged and beaten, let the players get creative' and given some guidelines. So the staff (whose staff bits were the Stardroids, which were semi-IC at that period of time, this was yeeeears ago) went out and made plot with those guidelines. We were given a lot of free reign by staff to play with the players, and it worked out really well with us advancing the 'metaplot'. Same thing happened when the game ran through its own version of the Megaman X4 plotline.
Incentives - numbers?
@Derp
My experience is also not a great cross-section of WoD MU*s, particularly the brand of crazy that an ALL THE THINGS ALL THE TIME game has in comparison to a focused game. Hell, my experience on a game with four competing factions (M3) is still not anything like that. We had people burn out, sure, but that's why you have to be willing to trust the newbies. Sometimes they make the best staffers. After all, we were ALL newbies once..
@Arkandel
If you want to incentivize it for the playerbase and need to do it as XP, it needs to be tailored to the game itself. For example, I have guidelines for TheatreMUSH about the amount of XP that can be handed out for a type of plot that is run by both players and staff. Low-game impact award 1 XP; moderate awards 2; and high game impact (with attendant high risk) awards 3. For GMC, I dunno, do it in the form of 1/2/3 beats or something. This is something that only the individual staff of a game can determine. If you feel staff need to get this reward, give it to them as a reward for their PC as a 'gimme' for running the plot; same as with the player who runs the plot if it's a non-staff run plot. They just gave you effort into doing the thing, so give them a cookie.
As far as staff, here's where my input's probably going to ruffle some feathers, because I really don't believe in incentivizing something that they should be doing as part of their duties as that category of staffer. It's like hiring someone to read and approve character applications and them not doing the character applications without being given a milk bone. If you have to incentivize it, that's a decision that staff of individual games need to make; there's no real way to incentivize in such a way that applies to every game, because every game has different needs and different methodologies and if it factors into the incentive, different systems as well.
Related to the topic of this and staff duties, because that's where this discussion boils down to: do you give people a reward for a job they volunteered for? People can argue 'not a job!' vs 'a job!' all day, but in the end, those you hired for staff are giving you some verbal commitment that they're going to render themselves responsible for that thing you hired them to do It may be often thankless, but they volunteered to do this thing and, if they're a good staffer, should be doing it to better the game for everyone. Incentive shouldn't matter, in my probably extremely off-kilter opinion, because you should be hiring staff who want the game to be better for the sake of the game itself.
But, again, I come from a much different type of MU* background and staffing setup than a lot of people here, from my history with things like superhero MU*s and themed games. There's also the factor of staff who, for wont of a better term, wear a lot of hats vs. a codified, delineated staff. When I staffed on M3, you were hired to do generally one, maybe two, things. Charstaff would do approvals for those factions and work with PCs, generally facheads, as well as theme staff on plots. Codestaff did code. And so on and so forth. Beast Wars Transmetals had a 'staff of many hats' as we all did everything, except 1) only I did code, and 2) one guy didn't do charapps but made up for it in the plot department.