I have the same issue - I'm an 8 on top with a smallish waist and very small boobs so anything that is going to cover my top half really *needs* to be an 8. However I have huge hips and a big bum, even fitting in to size 12 items on occasion but usually somewhere between an 8 and a 10 (annoying how so many places have different ideas of what an 8 is to other shops).
My general rule of thumb is - dresses tend to need to be a stretchy material and *have* to have zips (I also have really broad shoulders so dresses without zips don't go on over the top or over my hips). I can wear more restrictive dresses but I have to accept that walking becomes a "toddle" as my bum is restricted. But I *have* to wear dresses in an 8 due to the small boobs.
Anything floaty on the hips - I'd go for the smaller size too. But anything for the lower half I *tend* to go for bigger than ideal if I need to unless it's really stretchy material.
Otherwise for separates it's best to try and get each part in different sizes - things that come in sets all together may not fit great but if you can buy each half separately so get two different sizes it works!
And the number of issues I have with brands doing a 6-8 and a 10-12 (where on earth is the 8-10?! They have gaps in other sizes too I've noticed)
All I can suggest is try loads on and get a feel for whether you should risk the smaller or larger size in certain styles and types of material etc. Lh is good to get an idea on this as you can always use the excellent returns policy (although I guess depending on which country you're in at the mo that might not be so easy).
Adx