One of the the things I have learned over the past while working in any graphics program is to work larger than the final output. If my final image is to be 800x600px... I am sure to be working at 1024x768 or a little larger for most projects.
Reason? It allows me to apply a little more detail to an image and manipulate it easier when I have more pixels to work with. This is especially helpful when making masks and when trying to add, improve, or bring out small details in an image. The more complex an image... the harder it is to make pixel based edits on it when its smaller.
There are some cons to this process however. If you work an image that is extremely larger than the final output... you stand a chance of wasting time improving areas of the image that are lost when scaled down. You could also loose some detail if you scale down to far for the final image. There is not an exact formula for determining the best size to use for each project but... I tend to work normally at 125% to 150% scale and this seems to be a nice reasonable compromise.
Until you become comfortable with the sizes, I suggest experimenting with some large images. Reduced those images to different ratios and examine the details while zoomed in closely to see how scaling has effected it and you will then see what details will hold when scaled down for the final image.
Stay tuned for the next tip (Photoshop Tip #2)