Another one? Seems like they're coming even more these days. Gettin a little crowded 'round here...
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On the serious side - depth, thought and attention for what makes your post worth reading are the primary criteria I think of. A long post is pointless if it's just a bunch of jumbled words without much thought for if they attract any interest.
I can't really tell you how to do that, it just comes to certain people over time, and some slower than others (it's a snail for me). Practice is the key, I suppose. Try starting in free or an "open" casual roleplay and work your way up; free can have some good ones with standards and that work like casual in some cases. Advanced I would wait until you're comfortable with the particulars of the site.
If you haven't seen it yet,
this is a good reference.