Making a (stylish!) seasonal transition?

Composition:

Burgundy Jersey-knit Top (Grace Brand via Overstock.com)
Brown Cardigan (Calvin Klein)
Brown Leather Belt (Mom's closet)
Dark-wash Skinny Jeans (Forever 21)
Brown Leather Oxfords (Steve Madden via South Moon Under)
Yellow Suede Slouch Bag (Ann Taylor Loft)

Usage:
The months of March and April mess with me sartorially.  They devise nasty ways - blustery days, sudden thunder storms, random cold fronts -  to make me traipse up and down my attic steps, packing and unpacking the storage bins stocked with my seasonally appropriate apparel.

Since the closet space in our townhouse is tight, my husband and I divide our wardrobes into warm and cold weather clothing, and we store away the out-of-season garments.  Thus, when unpredictable spring weather hits, we struggle to find an optimal time at which to officially exchange winter's wardrobe for spring's.  Inevitably, just when we think heavy sweaters are unnecessary, March will surprise us with a 40 degree afternoon.

Such was the case with the above outfit.  I wore it last weekend while visiting my parents in southern Virginia.  On this particular mid-50 degree day, Mom and I had dressed to conquer the outdoor, downtown shops.  Originally, I had paired the skinny jeans and burgundy top with brown leather gladiator sandals.  But when rain began to fall and the temperature dropped, I speedily updated my ensemble.  Exchanging sandals for oxfords, and throwing on a cardigan in a neutral shade, I felt more prepared for the rainy chill.  Instead of scrunching my jeans around the leather sandals, I rolled them into a chunky cuff to show off the oxfords.  I also considered trading the yellow bag for a more subdued leather one.  In the end, however, I decided the pop of bright color infused the outfit with pre-season warmth.

These quick wardrobe fixes helped me transition a spring outfit into a late-winter one.  I was especially grateful for the  brown oxfords, and not just because they kept my feet dry.  These shoes will pair as well with skirts and shorts as they do with jeans and sweaters.

Still, I found myself wishing for more reliable transition garments.  In this case, a khaki trench coat would have been ideal, but I hesitate to consistently 'solve' the weather problem by merely throwing on a coat.  Nor do I want to wear jeans until mid-April.  At this time of year, I tend to cope with the weather's unpredictability by layering tops and cardigans and cinching everything with a belt.  But this type of outfit becomes boring quickly, and I'm on the lookout for more creative solutions.

Above all, I'd like to know what exactly distinguishes a "summer/spring" article of clothing from a "winter/fall" one?  Style experts suggest the division is blurring, and that aficionados should learn to mix cross-season garments more adeptly.  But for those of us with limited closet space and more drastically fluctuating climates, do we dress for spring by adopting more color?  By wearing the traditional shorter hemlines and lighter fabrics?  When exactly is it appropriate to make The Seasonal Switch?

Prompts:

• How do you create interest in your outfits during seasonal transitions?
• Which characteristics distinguish a "summer/spring" outfit from a "winter/fall" one?