|image from Pottery Barn|
Group your accessories in odd numbers (three or five work nicely!).
Group together items of various sizes - avoid putting all one size together. Mix it up by putting tall with short, wide with narrow, etc.
Try grouping items of the same colour together.
Displaying collections against a contrasting background makes them stand out.
Try grouping like items together as well. A seashell on the table, one on the mantel and one on the floor are "3 seashells in a room". Put them together and they're a "collection".
If you have a huge collection of something particular, display some and put the rest away - then rotate the pieces every few months to change things up a bit.
On a bookshelf, add some accessories so it's not just books - mixing together books and accessories adds a bit of interest and variety.
If you have mismatched or ugly accessories that you can't bear to part with, try painting them. A can of flat white spray paint can fix a lot! A group of mismatched accessories become a collection when they're unified by white spray paint - try vases, ceramic birds, pumpkins, etc. Anything you find can be painted to tie together.
Try one or all of these tips and see how your accessorizing improves. Let me know how it goes!