Have you ever opened up a letter from a company and saw that the letterhead seemed to take up half of the page? Now, sometimes these letterheads have been quite interesting to look at, so I supposed the company managed to achieve something with that, but it still does not change the fact that the thing is so huge it looks out of place and odd.
For those getting letterhead printing done this is the first mistake I see a lot and something you should be mindful of. The best letterheads are the ones that are not necessarily that noticeable. They give their information without jumping out at a person, and they do not draw attention away from what the letter itself is about.
This can be particularly annoying if you are trying to have a serious correspondence and your letterhead is the most noticeable thing on the page. It takes attention away from the matter at hand and a person ends up spending a decent amount of time looking at your letterhead rather than what you wrote in the letter itself.
The next mistake is kind of the opposite, and this is where the letterhead is so plain and basic I question why they spent money on letterhead printing at all. If you are going to create a letterhead at least add to it some kind of distinctive visual element. This might have to do with adding your logo to the top of the page or this might have to do with using coloring that suits your company.
No matter what you pick just be sure that your letterhead has a purpose and is not just there to be there.
Continuing along this same path also consider what information is on your letterhead. I have seen both too much and too little information before. A letterhead is often going to have the basic contact information for your company, not unlike your business card. This is so someone can take just the letter itself and keep it with them. They do not have to save the envelope to see what your address is, and they have your phone number if they need it.
Some people just put way to much contact information on those letterheads and kind of bog down the page with it. Others do not put much of anything, limiting it to just a phone number and address. Do you have a fax number or cell phone number as well for people? Make sure you put all the important information there.
Letterhead printing should really be part of every company, and I would consider the last common mistake of letterhead printing to be not getting it at all. Every company is going to be sending out a lot of letters, and it does not take much to design an effective letterhead, and make sure everything you send out has your basic contact information and your company logo on it.