If you're not sure a change is what you want, don't rush into it. Book an appointment a few weeks in advance and mull it over.
Have a consultation. Take plenty of magazine cuttings of the colour you want; a few pictures of the blonde, brunette, red (delete as applicable) you like will give your colourist a good idea of the look you want. I took a cutting of the new Haagen Dazs print ad and a picture of model Julia Johansen proving inspiration can come from anywhere!