Super Low Cost Touch Sensors
CONDUCTIVE FOAM
Neither Force Sensitive Resistors nor Conductive Foam have repeatability good enough to make a scale to weigh things, but both will change their resistance with pressure for a joystick port directly (100K range) or a "biased circuit" (see below).
The foam costs about $11 for 2 square feet, and is used to package ICs in transit. It's a 1/4 inch thick black foam that techs often get free when buying ICs and throw away. Put a facing like aluminum on either side and squeeze, voila, a variable resistor.
BIASING A SENSOR
The game machine or computer will have its specified input, such as zero to 5 volts, or 0-100K Ohms. The latter is typical of Logitec and other joysticks. However, your sensors may not directly work in this range. Foam will be lower, CDS cells won't go so close to zero, and other variations.
To set up more ideal match between your sensor and your game machine (called Biasing), you'll need to understand resistor networks at least, and even better, how to set up "Op Amps" (Operational Amplifiers) such as the classic 741. These can amplify, offset, or attenuate your sensor signal, even invert it if needed, and filter noise (60 Hz is everywhere). They can output either voltage or current and work in a range from plus or minus 5 to 15 volts.
The output of the Op Amp can in turn be put into a device called an "Opto-FET" such as the H11F1 (or H11F2 or H11F3) by Fairchild and others. Like the CDS/LED opto-isolator I described above, this device has an LED, but with an FET serving as the light sensitive variable. It comes in a light tight 8 pin IC package.
They're faster and less temperature sensitive than CDS cells, and cover a very wide range. But they don't go to zero ohms.
Another exciting topic tangent to this but beyond the scope now is "non-linear" electronics. Much of reality has geometric or logarithmic relations, including sensors, which can be dealt with in analog (like Op Amps) or in digital. Beyond the familiar binary digital computation there exists analog computation and profound matrix computation such as optical holographic computing, not very useful for games, but interesting I think.
So if you're using any new sensors in your game design, you'll make a safer product by using opto-isolation (usually can block more than 1000 volts of fault), but your software will have to take their low limits into account. H11F series goes down to around 100-200 Ohms.
