I wear button down shirts in all ways (tucked, untucked, buttoned, unbuttoned, tied at the waist) and find this gives the same shirt a lot of variety in how casual an outfit feels or not.
I also am a big fan of matching my handbag with shoes or belts. It helps add polish to my outfits. This also has me switching bags/footwear at least a few times a week. Sometimes I'll wear the same outfit and just change out the shoes and bag or belt, and things look completely different.
I am a big watch wearer and while I don't own many watches, I wear one every day. Sometimes I'll wear the same one for weeks at a time, other times I swap them daily with my outfits.
But I'm probably most known for my sunglasses and matching those to my outfits (black, brown, tan, red, pink, blue). I like the extra little dash of color/polish I get with sunglasses. Similar to watches sometimes I change sunglasses often, other times I'll go a few days in a row before swapping.
One thing I did last fall was try lots of different combinations with my athletic wear. Since athletic wear comes in lots of colors and I own a fair amount, it was both easy and fun to try out new color combinations. I not only got some new favorites, but I learned that matching sets in my athletic wear really helped with mixing things up.
Now matching tops and bottoms are on my shopping radar. I may not wear them together often (for example a seafoam green shirt and pants together) but having a top and bottom in the same color helps add a LOT of easy mix and matching options. It was the key to how I was able to mix up my athletic wear so easily when I did my different combinations experiment.
