I’m not an expert in graphic design, but there are a few tips that I can pass along.

Lettering and background separation

For example, when you’re designing a book cover that will be shrunk down to a thumbnail on a book site.  At thumbnail size, the lettering really needs to still be distinct and not fade into the background.

Unless your cover has stark black letters on a white background, contrast is something worth thinking about.

If you have a colourful background and colourful lettering then the two can become a bit less distinct as they are scaled down.  Edging the letters can help, or you could try a drop shadow.

Edging

The idea here is to put a high-contrast outline around your letters.  

• Take a visible copy of only your letters layer, and select anything which in not transparent.
• Now grow the selection, by as many pixels as you want for the outline (remembering this will all be scaled down to a thumbnail later).
• Fill the entire selection on your copy layer with a colour which stands out against both the lettering and the background
• Unselect the copy layer and move it behind your letters layer.
• You may wish to soften the edges sllightly with a blur filter.

Drop shadow

This technique is similar to above, but we’re not going to make a selection or grow it.

• Copy your letters layer and deasturate (so it is just grey).
• Use a heavy blur, for example a 20-pixel Gaussian blur.
• Further increase the darkness of the shadow layer.
• Now move the shadow layer behind your letters layer and using a movement tool, nudge the shadow slightly out from underneath the letters.
• Adjust the opacity of the shadow as necessary – you may still want the background to show through.

There you go.  Practise and come up with your own variations, and take a look at how the thumbnail would appear by scaling down yourself (then just undo to get back to your regular image).