When ordering online, I will look at the size chart to see if I can tell whether this maker generally runs large, small, or TTS. If I can see a general pattern, I will order accordingly with my usual sizes as a reference (e.g., I will order the smaller of my two sizes if the retailer appears to run large).
I have found that of all the measurements in the size chart, the waist measurements seem to be the least accurate. So I usually ignore those and focus on the hip instead--which is my problem spot.
I know that I generally wear one size smaller on top than on bottom, so I take that into account. If I'm ordering a fit-and-flare dress, I can use my bust size as the main point for ordering, because there's plenty of room to fit my rear end in a fit-and-flare. But if it's a form-fitting sheath, I will possibly need a size up. (I generally order two sizes for form-fitting bottoms.)
If the site is a retailer's own site (not a department store that stocks many lines of clothing), I will sometimes e-mail the retailer for exact measurements. J. Crew in particular has been excellent about giving me a timely response for measurements.
If shipping is free, I will generally order two sizes of things and then plan on returning one. I figure it's cheaper for everyone if I order everything up front and have only one shipping charge to and from, instead of extra charges to ship me replacement sizes.
As others noted, reviews can be helpful if you can pinpoint those written by people with your body shape.