For casual dress, I am in love with the long, cargo skirt. It's feminine, but it is playful and comfy and casual, and depending on the fabric, it can even be somewhat dressed up.
For dressing up, I am all about layering little jackets to cover my arms, since a lot of dress tops nowadays aren't perfectly modest.
Casual for men:
I totally prefer rolled-up sleeves to short sleeves and in the summer, lighter fabrics with long sleeves.
I really don't like flip-flops on anyone.
For dress up, like for Mass, I always prefer a jacket than than just a nice shirt by itself, but that's just me.
Now that I've got the ball rolling, I'll let some other people provide their suggestions and later I'll mention the stores I prefer for modest clothes for men or women.