I use Google Adwords - the ad boxes you see on the right hand side of the page. Results are linked to keywords you specify, and the budget you can afford to set per word, and total spend per day (higher budget, more chance of being in the top few spots). You pay per 'click through'. Your daily budget (of your choice) ensures no one can maliciously hike up your credit card bill.
To give an idea, I spend around £150 pm - I know others who spend over £1000, but with outstanding success.
Hope this helps.