Original Post
This is an adaptation of the insectroide's tutorial for a simple robot. Here's my version: Materials: AA battery case (Holds 2 AA's) radioshack: $1.69 AA case 2 3-Volt DC Motors radioshack: $2.99 each Motor 2 tactile sensors radioshack: $ 2.79 each switch Wire (This will last a while, guage isn't to important, over 20 radioshack: $ 7.00 (20 guage, semi-small) Wire 2 paperclips get 'em anywhere soldering iron find them for a good price solder different prices, go for rosin core I believe is what I use hot glue everyone has to have hotglue, a robot builders friend Procedure: 1) take your AA case, and remove the lid 2) glue the two switches onto your container, forming a 'V' at the front 3) glue your motors, one on each side, to the AA case 4) Wire up the components like so: 
5) bend the two paper clips into an antenna form, and glue them to the switches' metal sensor areas. (This is tricky, be sure you do this well, it may take some time) 6) take a small paper clip, and make a skid 7) glue this on the front of your robot, and adjust how high it pushes your 'bot off the ground until it moves in a straight line 8)If neccesary, raise up the back (I did this with thin slices of styrafoam. My bot would just vibrate unless I raised up the rear) notes: When soldering, make sure to heat up the wires being soldered, and don't glob solder onto the iron. I'm guilty, but it makes a kind of bad connection. If your robot rotates around the left motor, reverse the left motor's connections. If your robot rotates around the right motor, reverse the right motor's connections. HOW IT WORKS! Your batteries provide "Juice" Through the plus location. Electricity always will flow to a negative, or ground charge. The - of the battery is a ground. Now, power flows up to the switches, and if the switches are not pressed, they allow no current t oflow. The + connected to the two motors than flows through the motors, giving you forward movement! If the left sensor were to be pressed, it would have + power going to the motor, and the - connected to the other motor terminal would be "opened", and power would flow backwards through the motor. Now you have a reversed motor doing differential steering - the process of the outside of a turn's motor/velocity maker or whatever going forward, the inside slowing or reversing. This is a lot like a shopping cart, you press harder on the outside of the turn. Now, if the right switch detected, the same would happen, the right motor would reverse. It's as simple as that. Let her down onto a tile floor, and watch it go. EDITS: Added the processes 6 and higher added prices and link for switch and wire [Edited by - brandonman on March 7, 2008 4:02:55 PM]
5) bend the two paper clips into an antenna form, and glue them to the switches' metal sensor areas. (This is tricky, be sure you do this well, it may take some time) 6) take a small paper clip, and make a skid 7) glue this on the front of your robot, and adjust how high it pushes your 'bot off the ground until it moves in a straight line 8)If neccesary, raise up the back (I did this with thin slices of styrafoam. My bot would just vibrate unless I raised up the rear) notes: When soldering, make sure to heat up the wires being soldered, and don't glob solder onto the iron. I'm guilty, but it makes a kind of bad connection. If your robot rotates around the left motor, reverse the left motor's connections. If your robot rotates around the right motor, reverse the right motor's connections. HOW IT WORKS! Your batteries provide "Juice" Through the plus location. Electricity always will flow to a negative, or ground charge. The - of the battery is a ground. Now, power flows up to the switches, and if the switches are not pressed, they allow no current t oflow. The + connected to the two motors than flows through the motors, giving you forward movement! If the left sensor were to be pressed, it would have + power going to the motor, and the - connected to the other motor terminal would be "opened", and power would flow backwards through the motor. Now you have a reversed motor doing differential steering - the process of the outside of a turn's motor/velocity maker or whatever going forward, the inside slowing or reversing. This is a lot like a shopping cart, you press harder on the outside of the turn. Now, if the right switch detected, the same would happen, the right motor would reverse. It's as simple as that. Let her down onto a tile floor, and watch it go. EDITS: Added the processes 6 and higher added prices and link for switch and wire [Edited by - brandonman on March 7, 2008 4:02:55 PM]