Practice at demonstrating enthusiasm. "Fake it till you make it."

Also, sometimes the best defense is a good offense. So, if you commonly hear that you don't show enthusiasm for the job, then be proactive and say to the interviewer, "I've been told in the past that when I interview I don't convey my enthusiasm for the position very well. That has really stuck with me. I just want to be upfront and let you know I am so appreciate this opportunity to interview and I am *very* interested in this job - EVEN if for some reason I happen to give off the vibe that that is not true. I ___ [give a brief point or two why you want the job and to work for the company. Just launch right into it following the previous statement.]" Ways to show your interest nonverbally - leaning forward, eye contact, smiling. You may not be great in interviews, but being candid is often well received by the interviewer.

Hope you find this useful.