There needs to be a strict separation between the corporation and the people behind the corporation. As part of this, the LLC's finances need to be kept rigorously firewalled from personal finances — never pay a corporate bill from your own pocket, for example, or vice-versa.
You also need to follow the formalities of a corporation by keeping formal records (minutes, shareholder acts, etc.).
If you fail to do either of these things you, in addition to the LLC, can be sued, and you can lose personal assets. It's called 'piercing the corporate veil'. A great run-down is here:
http://www.nolo.com/legal-encyclopedia/personal-liability-pi...