Blues and greens for summer, as the colours seem to offset our sweltering summer temperatures, and then darker, warmer colours for winter.
I like to have a couple of linen sets (bed linen, cushions, curtains), that I change around - that way I use them for years, without growing tired of them.
Flowers, candles always brighten a room.
I also change knick-knacks regularly - once they've been in storage for a while and are then taken out again, they look like new