well if you want my advice when I used to work in men's suits for jcpenney, i had to be honest with all my cliets/customers. otherwise if i conned them into buying stuff they didn't want or that they didn't look good in, then i wouldn't get any repeat business. the key is like beth said, it's not so much what you say also as in how you say it. i even used to help girls figure out what to buy as well. in fact, there was this one girl that tried on a pair of dress pants that made her butt look huge. however, i didn't tell her flat out, "that pants makes your butt look fat or something like that." I just blamed it on the tailoring. i told her that from looking at the subtle stretch marks around the belt line and crotch area, it looks like the pants were made a bit smaller. which is true because different brands tailor differently as my experience in suits taught me. anyway, i hope that helps.
eta: also you can use lines like you want an outfit that'll insinuate your figure because if you wear pants that don't cause stretch marks, it gives you more of a slender physique appearance. using big and sincere words like that really got me a lot repeat business from clients too. lol.