The rules above are for everyone. What follows is more conditional, and is based on some assumptions about your goals as a volunteer. If you're reading this in search of advice (as opposed to reading this looking for ways to improve on it, which I also welcome), then you're probably interested in contributing more to your game of choice, and/or being asked to play juicier roles.
If you're a staffer or a full-timer, the first best thing you can do to help the game is to really work on learning the rules. There are a lot of rules that don't come up often, so only players with a strong expectation of interacting with them actually learn them. Essentially, you're expanding the roles you can be cast in by tackling the learning curve sometime before you're about to go in play. This is true even if you only expect to play combat roles. For a Rule of 3 example, learning the ins and outs of warrior orders (DtD) or Combat Disciplines (Eclipse) opens the door to more high-profile roles. Also, Plot will appreciate it.
The next best thing you can do, and for the same reason, is to spend some time soaking up the campaign's lore. This is a lot more important if you're focused on talky roles; while it's generally good even if you've asked Plot to cast you only in combat roles (as some full-timers and players do), it's a lot lower priority. Having context for players' responses to you in encounters, especially when the conversation goes outside the bounds of your briefing, lends depth and credibility to the character - it's another little element that improves the game for everyone around you. (For reasons of vanity, Plot also likes it when full-timers show investment in the setting and story.)
In LARPs that draw on players when casting some of their talky NPC roles, getting cast as recurring and/or important NPCs becomes a subtle striation between players. In my experience, it's not something that the gaming community talks about often or openly, but from time to time people mention a little disappointment that they are not asked to play recurring or high-profile NPCs. Everyone recognizes, I think, that there are a lot of considerations that go into NPC casting, but:
- It's fun to feel important, especially if you get to feel important around people your PC doesn't normally deal with. (There's nothing wrong with enjoying this, as long as you aren't a jerk about it and follow the Five Rules.)
- Getting an important role can be interpreted as Plot showing trust in you. It's satisfying to feel trusted and challenged.
On the other hand, if you're involved in damn near every storyline, Plot can't cast you in any role without spoiling your fun as a PC. A minor confession: I'm one of those people who did feel a little angst over never getting asked to play talky NPCs, in several campaigns. (So I started my own game.) (No, that's not the real reason.) If your PC's schtick is the same as the set of things you're known for doing well (for example, being super stealthy), it's even more difficult, because they'd be putting you in the scenes you most want to experience as a player.
Sometimes it really is too overwhelming to write the entire briefing necessary for a role, so Plot takes the easy way out and casts a staffer. Most frequently, Plot just needs as much certainty as possible that the person cast in a role will be at the events when they need that one character. I'm getting a little off-track here; the point is that Best Practices for NPC casting are complicated, and when they don't cast you, it isn't personal. If Plot has spare time to create NPC roles in response to your interest in playing them, that's great and all, but odds are, it's a low priority amid all of the other event-prep insanity.
If you're a full-timer, one of the best things you can do is to consistently let them know if you're volunteering for the next event. The majority of casting decisions are done before Friday night of an event - if they're surprised (but happy, of course) to see you, they definitely haven't written anything with you in mind.
Okay, my last bit of advice is awkward to deliver, and you've got to judge for yourself whether it might include you. High-profile combat roles (field battle bosses) and talky roles (the monarch, the archmage, whatever) are the kinds of roles where Plot wants someone with presence, confidence, and magnetism. If they don't know you well, Plot probably can't tell how awesome you would be in that role.
I want to stress that the converse is not inherently true. If you lack presence, Plot probably won't cast you in whatever role; however, not getting cast in whatever role does not necessarily indicate a judgment that you lack presence. As I mentioned before, there are lots of considerations that go into casting; it is mostly not about you.
I'm not an actual self-help writer, and I don't know One Weird Trick To Improve Your Charisma Score. What I do know is that if you show that you're engaging with the game's content and putting real energy into the roles that you do get cast, it makes a difference.
Kainenchen suggested that I include a brief explanation of why being a full-time NPC is fun, since from a certain perspective it lacks both the agency of a PC and the creative control of Plot.
- Let your villain side out for a spin without risking a character you've developed for months or years.
- Ham it up to your heart's content. Actually, this should have been Rule Five.
- You don't have to attend Plot meetings or shoulder any of the writing burden.
- If your character dies, no sweat! Plot will get you a new one at their earliest convenience. You'll get to try out all of the different combat styles and power sets in the game as a combat NPC. Talky NPCs have, if anything, even more variety.
- It's the least expensive way to be involved in the hobby. You don't pay event fees, and you don't need to buy costuming, weapons, armor... seriously, if you show up to an event with nothing but bedding, toiletries, and a few changes of basic black clothing (the universal foundation garment for NPCs), you're golden. Some amount of food will usually be provided to you. This varies somewhat from community to community, but probably not a whole lot - it's in the game's interests to avoid barriers to entry here.
- The camaraderie of the whole backstage team is fantastic. Staging an event is hard work, but when it's done, you have a lot to feel good about, and players appreciate full-timers immensely. (Trust me, PCs love it when there are enough full-timers that Plot doesn't need players for warm bodies.) There will usually be a solid round of applause for you on Sunday morning, if your game of choice has something like Closing Ceremonies.
I'm sure plenty of people have their own reasons for enjoying full-time NPCing, and I would be happy to have them add those reasons in the comments.
I hope something I've offered here helps to make your time spent NPCing all the more rewarding (or reminds you to show your appreciation for the NPCs, while you're PCing).