I don't cover up completely. My hair and my arms are uncovered and I generally don't wear super tight clothes, unless it's to par-tay, then that's another ball-game in itself! I have a basic rule for myself; if I'm wearing tight pants then usually my top is loose and if my top is more fitted then, my pants are loose. I also always have a scarf in my bag in case I want to wrap it around my neck if I feel uncomfortable or just want to cover up a little more (on the GO train/bus this is a must as it's always freezing in there!).
Going to shop for summer clothes is such a huge challenge! I feel like everything is micro short or has a huge hole in it...drives me crazy! I usually always end up liking a shirt and then flip it around to find out it's backless or see-through or has a huge slit running through it....sad times.
I don't tend to have a huge summer wardrobe; it mostly consists of t-shirts (so fashionable right?). If i do like something then I buy it in every colour possible so, that I have no regrets later. I recently went shopping and bought some great stuff at H&M, Forever 21, Aritzia and Club Monaco. I feel more stores nowadays cater to absolutely everyone so, the variety is huge. Even stores such as Mango, have a "Muslim-friendly" line that they launched in Ramadan.
So, how do you stay modest in this crazy heat wave we are going through? dresses and skirts are your best friends! Opt for lighter colours and breathable fabric like cotton. If you do layer then once again opt for breathable fabric for your layering. Try to always remember your proportions so if you do wear looser clothing and don't want to look too boxy; try wearing heels as it will elongate your entire body or add a blazer to give you some definition. Long cardigans have been a great fashion accessory for me. It covers you up while giving you a fashionable edge.
Staying "modest" is difficult but it doesn't mean that you can't be fashionable and trendy. Try to find your own medium and see what works for your own body and style. Don't let society or others define your sense of fashion and restrict you in their rules. Most importatntly, stay cool!