I generally agree with Mafi. Long posts are great and all, but forcing people to make them isn't. I myself struggle to flesh out paragraphs, preferring to get straight to the point, though I do find that as I play more D&D and do more RPing in general, I'm able to make far more out of much smaller periods of IC time.
The way I've always done it is simply having standards: if you're clearly putting no effort into your posts, you're out. It's alright to occasionally post something brief just to keep things moving, but continuously failing to do anything of real interest wastes everybody's time.
I fear I'm not being entirely clear, but assume you'll understand my basic point. I'll blame it on my time of writing being 3 AM.