Simple Movement -- Blender 3D Game Engine

Add a Motion Actuator -- Servo Control

The Servo Control tries to maintain a set linear velocity.  It applies as much or as little force needed to maintain the set velocity.

Buttons Window menu >> Logic (F4)
Actuators Add
Motion Actuator
Servo Control

Servo Control Settings: use game object (local) Y axis for forward movement

Enable the Limit for the Y axis and give it a Max and Min force to apply.
If Limit is not enabled the Servo Control will use whatever force is necessary (ie no limits) to try to make the linear velocity stay at the set amount.  If you run into a wall and can't move, the Servo Control will keep applying more and more force.  You might shoot through the wall or when you release the key, slide backwards.

L (local) button enabled

linV = 10.0
Limit -- Max: 200.00 
Limit -- Min:  -200.00