I also have this problem!! In fact, I went shopping today looking for thermals and had a time and a half finding shirts that weren't tight across my chest. (Found some fantastic and super cheap ones are Rue 21 though!) I'm a college student so I'm always hunting for cheap plus-size clothes!
I've found that the best fit for me is relaxed straight leg. I have pretty big thighs and hips - my grandma has always called them "child-bearing hips"

- and that fit tends to be generous in the hips and thighs but tapers enough in the legs to successfully keep my jeans from looking like parachute pants. I've got one pair that fit great from JCPenny but they were a splurge at $40. I love the relaxed straight leg from American Eagle though, and while they are usually pretty expensive, if you catch them on sale you can get them for $25 a pair! Also, I've gotten Wrangler relaxed straight leg from Walmart a couple times for $17 a pair and they fit almost as well as the American Eagle jeans.
Not sure how you like your shirts to fit, but I wear a binder that is not exactly super effective and so I like my shirts to be loose in the chest but fitted everywhere else. Best places I've found shirts:
T-shirts: Walmart, because I can get them for like $3 and they fit perfectly; Rue 21, because I like v-necks and their v-necks aren't very deep and so they won't show the top of my binder; and Old Navy, because they have a ton of styles and colors and are almost always on sale!
Thermals: Like I said earlier, Rue 21 has the best thermals I've found that aren't too tight in the chest. Kohl's also has good ones. I don't know if thermals are all that important to you but they're a major staple in my fall/winter closet.

I'm not a sweater kind of guy, so I've got no suggestions for those. I do recommend staying away from button-ups though, as I've always felt that, at least for me, they accentuate my chest a lot and don't fall right on my hips to help hide anything there. Ummm, no other shirt suggestions come to mind, but I'll add more if I think of any. ^_^
Dunno if this is something you're looking into, but I recently found an awesome brand of boxer briefs with virtually no pouch and they don't ride up/roll down like a lot of other brands do because of my curvy hips and thighs. They're Uniqlo seamless boxer briefs and I think you can only get them online in the U.S. but I have no idea if that's actually the case.

I hope this helps at least a little!