My referrer log shows how people find this site. A large number are through searches. Some are unpublishable and others are just odd. This one showed up and is so easy to answer that I decided to answer it twice - with conflicting answers:
does buying more lottery tickets increase your chance of winning? - Yahoo! Search Results
The simple answer is yes. If they sell as many tickets as there are buyers then with each ticket you buy you increase your chance and decrease the other person's chance. If there are two buyers and you each buy one ticket you have a 50% chance of winning. If you then buy one more, you have a 66% chance of winning. The equation behind this holds for any positive number.
But the complex answer is that in a lottery money is siphoned off to pay for the companies which think up, advertise and run the lotteries, not to mention the states taking a portion for schools so statistically a lottery is a losing bet. In the big picture buying more tickets merely means you lose more.