If there was one lolita fashion discovery I wish I had made sooner, it's how useful boleros (and cardigans) can be!
Don't feel like wearing a blouse under your JSK? Wear a bolero over it instead! Especially in Summer, it's a life-saver! And it's usually much easier to wash a bolero than a blouse too (and requires less ironing).
Got a dress that looks funny with a blouse under it? Then wear it with a cardigan or bolero over the top instead! Some dresses are hard to co-ordinate with a blouse because of the shape of the collar or sleeves, so boleros or cardigans can solve this problem.
What about if a dress doesn't fit properly? Or has damage to the collar you can't fix (like make-up stains)? Again, boleros and cardigans can help you get more wear out of these dresses by hiding a multitude of sins.
So in conclusion, buying long and short sleeve boleros and cardigans in a few colours (black, white, cream, pink) can extend your wardrobe by a lot! Personally I'm working on completely my bolero and cardigan collection all the time, because they are just so useful:D