Good answers below. In brief, the market is very hot for lower- and medium-priced homes in Centreville. You have to make sure you get an offer in early, that it's price competitive, and that your other terms and conditions are attractive, too.
No, it's generally not investors. Investors don't compete against "retail" buyers such as yourself, because retail buyers are willing to pay much more than investors. That happened a lot in Manassas and Woodbridge about a year ago. Investors were picking up a lot of properties. Then, around about March 2009, retail buyers decided to re-enter the market. That meant that Manassas townhouses that investors had been buying for $80,000-$100,000 suddenly jumped in price to maybe $130,000. They were still great deals for retail buyers, but no longer for investors.
You're mostly competing against other people like yourself who are looking for affordable housing.
Joseph describes one technique that can work. You might also find investors selling properties--not ones they've just bought, but properties they bought a year or two ago as "buy and hold" properties or to offer as lease-options. Some will use Realtors to sell their properties. But others won't; they do it privately and you'll never see the properties on the MLS. (Or, around here, it's MRIS.) There are also some Realtors (I'm one, and I know another extremely good one in Centreville) who work largely with investors. In many of these scenarios, you're eliminating much of the competition because the property isn't on the market in the MLS.
Ask your Realtor for additional guidance. But I suspect that, to be successful, you'll need more than: "Well, all we can do is keep making offers and hopefully we'll get lucky." Not saying your agent will respond like that, but if he/she does you may want to pursue a more successful strategy.
Hope that helps.