I think the biggest mistake that any business is making, but especially a smaller business, is not thinking about archiving as an integrated business activity. Instead, they're thinking of it as something you buy. Of course, you have to buy products to get archiving up and running, but at the end of the day, the real value of implementing an email archive comes not from what it is, but what you do with it.
So why do you buy archiving? Do you buy it for IT reasons, because you wanted to reduce the volume of email that you had to deal with in your email server? Or do you buy it to be able to search and discover email and produce it if the company gets sued? Or do you do it for productivity reasons, to help people better access and manage their email?
You have to ask these questions and think about them in terms of which products you are going to select, because some products are just naturally better at one thing than the other. Some of them are really focused on the IT solution, some are really focused on the legal solution and some are in between, just helping general productivity. The biggest mistake companies make is just going out and buying something they think will work -- usually from an IT perspective -- then coming home and finding out it doesn't meet the expectations of the people in the rest of the company.
Check out the entire Email Archiving FAQ.