I'm not a designer, but just some thoughts, mostly spurred by looking around Smashwords...
If in doubt, keep it simple. It's better to have a simple but neat cover than one that turned into a jumbled mess as a result of attempts to make it "spiffy".
If you start with an image the same format as the cover, and you have to squish your title awkwardly to keep it readable, don't do it. Find/create/commission a different image, or edit the image to make part of it low contrast enough to serve as background, or something.
If you look online for images to use as cover illustrations: Respect copyright. Google image search is not a clipart site. Most images posted on the net are
not in the public domain. It's only free to use if the creator of tha image says so explicitly.
If you resize an image, resize it proportionally.
Don't have the text touch the edge of the cover.
For the love of KITTENS, don't add lens flares.
If you're publishing a series, try to use the same general layout and style for all covers, for recognition value.
There are always exceptions, but only if you really know what you're doing.
