http://kevin.org/frc/
Use these sensors to keep track of your location on a virtual grid.
Keep the calculations to the basic right triangle trig calculations.
I am still thinking about it but the best way i think is to know the angle <90 of the robot direction to parallel of the field. This is done by adding and subtracting the gyro values. Then use sin and cos to calulate the change in x and y based on the distance you traveled in that direction. The accelerometer will help you with this.