Best principles: (echoing most of whats been written)
-Set dates, times, "point of contact" person for work efforts
-hold them regularly (many but short and defined)
-follow through and be sure to be there those times you've comitted to.
-keep the mood positive during the work
-don't talk about whos not there during the work
-accept help from the helpful, but don't try to "buy" your help too much
-take photos during the work and post them/tack em up somewhere
and, when one of the core 6-8 helpers gets frazzled and the rest of the usual crew senses they are about to go on a rampage on your local forum, club board - with a rant... stop them if you can - tell them to take the next project or two off - say... "we've got it covered"
Fist shaking at the ingrateful seems not to work out surprisingly.
If possible, schedule different workdates for different days. If they're all scheduled for Saturday mornings, you might be missing out on potential helpers who are never available at that time.