I am having this same problem, Julia. My issue is that it will be blazing hot outside and I'll be miserable and sweaty when I'm out in the heat, but then when I go inside, I will be freezing from the A/C. Will this be an issue with you as well?
I have already gotten started on putting together my summer wardrobe since it's getting warm here (in the 80s now). From Angie, I learned that knee length walk shorts or skirts would be most flattering and appropriate to the smart casual look. Then I decided on the requirements for summer tops based on what I think would be most flattering for my body type: ie, must be sleeveless, V-neck or low scoop neck, empire waist, but no pleats, no cap sleeves, no puff sleeves, etc.... Then, when I went shopping, I didn't have to waste a lot of time trying things on that I already knew wouldn't work. It took a while to find what I was looking for, but I managed to find one great top at Ann Taylor, a nice one from bluefly, and 3 nice tops from Ann Taylor Outlet. The tops from the Outlet are not super nice quality, but they were the only things I could find that fit all my requirements, and they only cost me $17.00 each, so if they only last me this season, it's not such a tragedy. Of course, I'm still keeping my eye out for nicer quality items, but at least I've got my bases covered. I think that with summer outfits, since layering is kept to a minimum, accessories like jewelry or belts/shoes become more important. They are probably what are going to add the most interest to your summer outfit, so don't forget about those.
Deciding what to look for ahead of time is really really helpful, I would definitely recommend this, because it's frustrating to not find what you want, but I think it's more discouraging and frustrating just trying everything on without having an idea of what would work for you (my old way of shopping) and then deciding there's nothing out there for you.
Now, you can still layer in the summer if you want to, but the trick is choosing very light fabrics to layer. Like, some of my sleeveless tops can be worn as a single layer, but look much nicer and more interesting with a lacy cami or something layered underneath. This helps when you go into air conditioning as well. Another thing to consider is maybe a light, short sleeved cardigan to have in your bag or something to put on when you go inside. J. Crew has awesome short sleeved cotton cardigans in fun colors. I only have a black one (boring) but I'm thinking of purchasing a second more colorful one:
http://www.jcrew.com/catalog/p.....=cat301172
My cardigan has worn very well, no shaving necessary yet.
So, I guess to boil down my really long post, I would say that for me, a Summer Smart Casual formula might be something like this:
Crisp Walk Shorts/Dark Denim Walk Shorts/Dark Denim A Line Skirt + Knit Sleeveless top (+ Cami) + Short Sleeve Cardigan/Blazer (Optional) + Interesting Accessories + Dressy Sandal/Ballet Flat
Dresses are also wonderful alternatives, because it's just one thing, dress it up with some interesting accessories, and you're ready to go.
Hope this helps and doesn't just overwhelm you!
