The faxyourMP people make efforts to collect info on who is replying and who isn't, with a view to publishing the figures.

The point isn't really getting a reply, its nagging them into taking some interest, in the hope that that can make other MPs see it as a real issue.

If they don't answer, or ifvtheybjust send an acklowledgement, send them a series of follow-ups to keep them in touch with developments. And if they do answer, send them a reply explaining why they've got it wrong or - if you are unusually lucky - thanking them for getting it right.