Best stores, in my experience. Originally pear shaped, but now I'm much more masculine. Don't know what I weigh, but I'm about 6'0 now so I definitely am a big guy.
Shirts:
* River Island (lethal for binders, sometimes the neck holes are massive, but otherwise great, good sales)
* American Eagle (Good sales. Go up to really big sizes. I wear a small in their shirts, no joke. Not to mention comfortable)
* Diesel (the thicker shirts only. AVOID the slim fit if you want to pass... they're hell. I got some as a present and the thicker shirt is one of my favourites, but the thinner one makes me look horrific)
* Levis (I have a cardigan from there and it was very cheap on sale, fits perfect, and looks great)
* Penneys/Primark (If you're from Ireland or the UK. When I came out, I went there because you can get shirts for less than 3 euro and other essentials. I pass best in their shirts for some reason. They're comfortable, last for a decent amount of time, and ridiculously cheap)
* H&M (I love their button up shirts. They're great quality and very comfortable. You NEED to iron them though, or they get weird quick)
Hoodies/Pullovers:
* Superdry (Fit weirdly - they're pretty straight, but that means you pass damn well. Comfy, warm, good value. They ball up a lot though on the inside, so your shirt is left covered in fluff which is very annoying. Nice range too)
* Hollister (one of my favourites... so cheap, so comfortable, so cool... but the quality isn't great sometimes. I have some stuff that looks good as new after a year, and others that started to look awkward after a few wears. Great sales though)
* American Eagle (Again, cheap, big sizes and all if you need it... I only have Summer hoodies from them and a denim jacket, but damn they're nice and the designs are cool too)
* GAP (Got some nice, cheap hoodies from them. Sizing is wacky, but they're nice and comfy. The sales are good sometimes since the store isn't doing too well)
* Old Navy (when i go to canada we spend a lot there. Cheap jumpers and t-shirts, good quality, all you need really)
Jeans/Pants:
* Levi's (only jeans that fit - you can get custom fit as well, so you're not stuck if you're over 34" inseam and have thick legs.)
*Aeropostale (picked up some nice joggers here. They last well, they're fairly comfy, but can be too warm.)
* Produkt (No idea if this is available outside my humble homeland, but this company don't seem to have any website... cheap and good quality clothes, long legs on their joggers so an A+ from me)
* Nike ( good joggers, price sucks unless it's on sale, i got some of their classic combat ones that are kind of like swimsuit material and I love 'em. The jersey/flannel/cotton whatever they are joggers are a huge pain in the ass though, since they're very straight leg and stiff)
Try to avoid bright colours or patterns. Maybe I'm too European, but the "tumblr boys" have it right in only wearing monochrome and dark colours. My whole closet really is only dark red, dark blue/navy, black, grey and white. Get a few, high quality pieces that all compliment each other and they'll work out fine. Also, push up your sleeves to the elbow with long sleeved shirts, but NEVER buy cuffed sleeves on t-shirts (this is more popular with girls and does nothing for passing, as I quickly learned). I luckily don't have to do this, but try to make yourself more "flowing" - no short pants, no rolling up the bottom of your pants, and not drawing attention to your socks. It makes you look taller. Unless, of course, you're very small and have to roll up your pants - in that case, only roll them up to the bare minimum, or you'll make yourself look even shorter.