Contrasting bright clothing. Bright green and red; bright yellow and red; orange and cyan... all these are hard not to see and tend to stand out in both bright sun and shadows.

Avoid black or other solid dark colors... it never ceases to amaze me that bike clothing manufactures even make black clothing. On my commuter I also add bright swatches of cloth to the bags to make the whole bike appear larger. I use strobes at dusk.

For night time wear, lots and lots of reflectors and of course bright lights.