Well, I'm nearing the end of the first Virtual Track Day (VTD) series, and I have to say, I think I've been very lucky: take up was good, and there has been very little drop off. I think this has a lot to do with the concept itself: while there is clearly one person who gets fastest time of day, there are plenty of other ppl competing in the other classes who get out there and enjoy themselves. They don't just drive around with their thumb in their bum and their mind in neutral though, they are still competing with the ppl around them.
The classes are a double edged sword, though, as they are very complicated, almost indecipherable in fact, and can lead to some confusion. I am trying to find a simpler class structure for the next series.
Secondly, it's a pretty friendly series (although Goose can be something of a Rottweiler when he wants to be

), and unintimidating in its very nature: I think everyone understands that you're meant to enjoy yourself, help one another out on set-ups, take the mickey a bit, have a bit of banter, and, if you've chosen a rubbish car, trail around at the back of the field looking at the wildlife (honestly, I don't know what else Scaff gets up to in his '70 Mitsubishi GTO

). If all you can do is win or lose, there's bound to be some drop off...
Thirdly, I think we gained ppl by allowing a certain amount of restricted tuning, and encouraging ppl to play with set-ups. We benefitted greatly from the presence of Scaff and Dave_George, who took the series as a tuning exercise as much as a race, and gave some great advice to everyone along the way. Without them, the series would have been half as much fun, and fewer ppl would have got to the end.
You're going to hate me now, but I've never taken part in an OLR series before starting the VTD: why not? It's too intimidating, and I don't feel I'm good enough. Admittedly, I'm not a good example, as I'm not terribly competitive by nature (I know there are plenty of ppl out there faster than me), but I'm still not going to take part in something if I'm only going to lose, and there's no other point to the racing....
My advice? Make merely taking part enjoyable; encourage
all the participants; try to build some kind of team spirit; try to make sure there's some interest for everyone; plus be efficient and fair (as said above).
That's the organiser's perspective, I'd love to know what the participants think...