Zak: Generally if you are short-waisted you’ll want to make your torso look longer, so that’s what a lot of these tips will assume (by the way most of these tips are from the men’s section of the What Not to Wear book and various articles from askmen.com). Making your torso look longer will often also make you look taller, if that’s a concern. A lot of guys opt for shirts with vertical stripes for this reason. It’s also a good idea to avoid tucking in your shirt or wearing shorter shirts in general, you can easily make your upper body look longer and conceal your hips by wearing a longer shirt. You’ll also want to avoid breaking up your look with contrasting colors or patterns (for instance, wearing a shirt with vertical stripes over plaid pants or brightly colored chinos with a dark shirt) because you want your waist to blend instead of stand out. You should also try to wear your pants at your hips instead of your waist.
For more info you can check out How to Hide Flaws With Clothing from Askmen.