You should keep your business and personal funds completely separate. The business should have its own checking account and its own credit card. This seems simple enough but new businesses can run into problems.
The first cause is usually under capitalization. You should forecast how much will be needed in start up costs. This would include the purchase of equipment, machinery, computers and other hardware. It would also include office supplies, software, postage and other smaller items. You will also need to anticipate your monthly overhead including phone, advertising, utilities, rent, etc. Other start up costs will include professional services from an attorney, account, business consultant, etc. If you sell a product, you will need a sufficient beginning inventory to meet the needs of your customers.
The business should be capitalized with enough money to support it with out any sales for the first six months. This amount could be any where from $2,000 to $20,000 for a small service or retail business. If you are beginning a construction or manufacturing business the costs will be considerably higher because of the costs of vehicles and machinery.
The other related cause is cash flow. Often businesses do not keep enough money in the business account to pay their overhead. This might happen because their account receivables are building up, i.e. their customers are not paying their bills. It might also happen when significant bills are due, e.g. estimated taxes. Cash flow problems cause the majority of first year business failures.
In general, make sure you are sufficiently capitalized when you start and keep a sufficient balance in your business account to handle any known or unknown liabilities.