Just 2 cents from a relatively new rider/racer on the scene based on recent experience ...
1.
Slower riders exist. Deal with it, it's part of the race. As part of your race strategy (and without doing it insincerely), I would think it prudent that you be polite and be as nice as possible to whomever you're passing. This includes (as Gonz has suggested) having a small conversation with them, especially on the longer enduros where everyone is hurting. He/she will appreciate the momentary distraction from the hurt and any encouragement you can provide. I appreciate the distraction the conversation brings as well. They will be more willing to go out of their way to let you pass. Even moreso if you encounter them again. A simple "Hi again" reinforces this and without even asking, you'll find them letting you pass as soon as they realise it's you.
2. Most riders are aware of the importance of keeping
momentum up technical pinches. As a follow on from point 1 above, if a slower rider is in front of you and is likely to stop you from keeping that momentum, too bad, deal with it. Not everyone can be as awesome as you just because you can power up techie climbs.
3.
I will go out of my way to let a faster rider through. Now this is not for everyone, however .. I will usually ask "Do you want to pass?" even before he/she calls "Track". If I get a "Yes", I will immediately start thinking where ahead would be the best place. I'll respond with something like, "No worries, let me just get down this techie descent, then you can go on my left/right/inside/outside just up here on the fire road / after the next turn / etc". More often than not, you're remembered for your courtesy and more patience is provided to you should they need to pass you again. But beware, if I don't get a sense of gratitude, don't expect the same level of courtesy from me ever again. Ever!
4. I reiterate,
have a conversation. After that short conversation, it needs to be clear to both the passer and the passee how the passing is going to occur. A coordinated effort makes for less incidents.
5. Lastly, (in relation to just
general riding), unless a track is signposted as one way only, don't assume that it is. At the risk of being shot down
.. I refer in particular to The Oaks single track. I frequently like to ride UP the track from Glenbrook to the picnic area. Now there's obvious peak times (like mid saturday/sunday mornings) when I wouldn't venture up there knowing that there'd be a multitude of people barrelling down. But when I do venture up, I make a concerted effort to listen out for oncoming riders ready to yell "RIDER!" straight away. And contrary to the general rule of thumb that the uphill rider has right of way, I'm aware that I'm doing something out of the norm and I will stop, hop off the bike if needed and clear the track for the oncoming rider. What I don't appreciate is the number of snide comments I've received from oncoming riders recently, who mumble under their breath things like "Yeah, real smart". Just so I can get it off my back, P*SS OFF! It's a shared track (so you should be ready to stop if there are walkers) and it's not one way. So take your attitude elsewhere but here.
But as a lot of people have mentioned, some people are just *ssholes. And as much as we'd like to keep them out of our sport, we need to accept that and just do what we can to keep up the stereotypical friendly attitude associated with MTBing.