They are an important business stationery product. If one is not designed well, it can lose a lot of it's function and even make your clients eyes bleed if it's not designed well.
A co-monkey of mine wrote a blog about how to create a good custom letterhead and you should take that advice into consideration.
Here is the link
One thing that you might also want to do with the design of the letterhead is to consider what kind of colors you are going to use if you plan on using a design that takes up the whole page. If you want to use a design that's going to take up the whole front side, you should make it very faint. Adjust the tint of it to roughly about 10% so it just barely shows up because if you have a darker object, the type above it will most likely not be visible to those who will be reading it.
Remember, think about the end user. Who is getting the letterhead and reading it? Cater it to them.
If you can't think of anything to design or if it just isn't your cup of tea, PurpleMonkeyPrinting.com has Graphic Design Services for your convenience. We also have some great pricing for them and we can even print them for you. So come check it out!