1. Money
Keep the company money separate from your personal money. Not only does this mean opening up separate accounts, but also giving yourself little reminders that keep the money separate. This includes using different banks for corporate and personal accounts so you'll know which is which just by seeing an envelope. You should even choose different check colors so you don't get them confused, even accidentally.
On the same theory, get a separate credit card for your business. Use it only for business expenses -- no exceptions!
2. Financing
If you create a corporation or a limited liability company be sure that the entity has adequate financing. That is, the company should be able to pay its own bills from its own checking account and should not take on obligations for which it cannot pay. This may mean that the entity needs to take a loan from you or obtain funding some other way (venture capital funding, angel investors). However the funding is accomplished, appropriate paperwork should be completed, particularly if you, the sole owner will be pouring your funds into this proposition.
3. Corporate formalities
A company must observe corporate formalities, meaning that you must have annual meetings and minutes from those meetings. If you want to take action, take a vote, create a resolution. Issue stock certificates. Run your company as if there were 500 stockholders and not just one and that each position of officer and director were filled by different, unrelated people. The treasurer is the one who should be in charge of he money; the secretary should maintain the corporate minutes. All this may seem horribly obvious, but these formalities and distinctions between you and your entity can quickly fade away when you are pre-occupied with actually doing what your business does. However, you cannot forget that you have to run and maintain the business part of your business in addition to whatever your business does.