When things are slow, I just try to take the time to focus my attention on other things - new stock, tweaking titles, updating photos, descriptions. Sometimes these things seem to help, sometimes I'm sure it's just filling time, but IMHO never a waste of time to try different things or make improvements. 

NPBs can be a nuisance, but I just try to make sure they take up as little of my time and attention as possible (which I personally do by sending one enquiry around day 4, a firmly set time-frame for extra days if they respond and ask for it, or open the UPI if no response to my enquiry after around 36-48 hours.

I have a pretty clear-cut MO - as it were - for things I am or am not prepared to do, so whenever a customer enquires about things like discounts and I'm not prepared to offer any (I am able to provide some items in bulk quantities, so will occasionally be able to offer a discount), if not, then I respond with a simple message that lets them know I am not offering any discounts on the item price, but they are welcome to take advantage of the combine postage discounts I offer.  (<<<had to edit in the wink  ).