Accessories are clearly the easiest way to change it up - a statement necklace adds some visual interest and can dress up any outfit. A scarf has an easy vibe to it and is a great way to add color or pattern. A brooch or flower pin can make an old blazer look fresh. And my hard, fast rule to any jacket that looks a little frumpy is "put a belt on it!" A jacket that has a bit of a defined waist can look completely different with a colorful or studded belt (I love the obi wrap belt from The Limited!).
Some of my favorite places to look for affordable jackets are The Limited (especially the sale section!), Target, Gap, and Forever 21 - their new Essentials line and any of the Love 21 pieces fit more like misses than juniors. I can also occasionally find a deal at Goodwill. It's easy to look there since blazers are all hung together.
If you're interested to see other ways I remix different pieces, check out my links to 30x30, Take One, and Friday Favorites.