There is no secret to keeping your basement floor dry.
Per your question, you are off to a great start by installing gravel and a vapor barrier.
The next step is to manage ground water. I typically recommend and specify the installation of a proper drain tile (perforated pipe with a geotextile clothe sleeve) system. Drain tile should be installed both at the exterior perimeter of the basement with bleeders and interior perimeter drain tile, all connected to a sump pump.
Also to minimize moisture and or condensation build up on the concrete floor, you should install a whole house dehumidifying system. This will maintain proper moisture levels though out the home increasing the efficiency of heating and cooling system while reducing mold, mildew and unwanted pests such as dust mites and other bugs drawn to water.
Best of Luck,
Harold J Remlinger, AIA, LEED AP BD+C DesignTeam Plus, LLC
For more information:
Read "What can we do about water build up on our basement floor? We had a french drain put in and it worked for the first 3-4 years." a Q&A answered by Edward Wright.