When signing documents on behalf of your company, be clear that's what you are doing. There are various ways to do this, but the best is to specifically note this and to note your role in your company. For example, if you are a manager of your LLC, you would sign as follows: "John Doe, Manager of XYZ Company." If you aren't sure what your official title is in your company (you must not be using AfterIncorporation.com . . .), drop the title and sign as follows: "John Doe, on behalf of XYZ Company." It's better than nothing.
Admittedly, many documents you sign won't expose you to liability, regardless of how you sign them. When you sign receipts for the purchase of office supplies, letters or time cards, there's not a lot of exposure here. But there is still value in taking the formal approach as much as possible. By doing so, you are in effect drawing a line between what is personal and what is business. It shows that you are taking that division seriously. This has benefits for tax purposes, and heaven forbid your company goes under one day and creditors are chasing you, you will be in a stronger position.
For more on this topic from the Bongiovi firm in Las Vegas, check out this article.