The most obvious reason for your daily budget exhaustion is the fact that your daily budget can’t accommodate the amount of ad traffic your ads receives per day. There are three ways to prolong the amount of time your ads run each day. The first step will be to optimize your account by refining your keyword list so that it contains only the most targeted terms. Ones you have done this you can also lower your CPC bid. This will reduce your ad ranking as well as the amount you pay per click. If you don’t feel comfortable with the first two options you can always just increase your daily budget.
Your ads Delivery Method settings can also have an impact on your daily budget exhaustion. Certain times through the day may yield more valuable traffic for your ad than other times. With the ‘Standard’ delivery method, the Google AdWords system determines the most effective time period to use the majority of your budget, rather than distributing it evenly through the day. An ‘Accelerated’ delivery setting on the other hand will show your ads as often as possible. This may deplete your daily budget more quickly than you would like it to. By chousing the right delivery method you may still be able to stay in your daily budget limit.
Dynamic changes in web content, user behavior, news items, trends and current events can also dramatically influence the amount of impressions and clicks you receive though the day. Although it is important to keep in mind, it is mush more difficult to pin point.