I think the previous answers were good enough. Generally ask open ended questions, never leave the reader thinking that everything has been answered. Try your best to engage your reader in the conversation and give them a chance to respond.
If you want some awesome tips on blogs and commenting, I highly suggest picking up some pointers from
Liz Strauss at Successful Blog - Thinking, writing, business ideas . . . You’re only a stranger once. Liz Strauss is a pro at commenting/receiving comments.