How to increase tension on potentiometer?

That I know of it can't be. Replace the pot with one that has a bit more "drag" to it.

What type of Tele is it and how old?

 

You can remove the knob, place a felt washer over the stem, and put the knob back down with pressure as you tighten the hex screw. That works for the knobs on my pedals.

 

Put an O ring over the shaft and then put the knob on pushing it down onto the rubber washer and giving it more resistance with the body of the guitar. This works best on the knobs with a set screw. The only drawback maybe increased height of the knob.
Same thing rhodco said basically.

 

Damn, I learned a useful "Bubba mod" for pots. Thanks guy!

 

You can sometimes make them stiffer by filling the inside the pot body with automotive dielectric grease - get the kind that comes in a pressurized can and inject it into the small hole in the back of the pot where the leads come out of the body. You should be able to do this without removing the pot from the control plate.

 

There's some good advice here. I have a distortion pedal with knobs that move if you breathe near them.

 

I used the o ring trick with great results. My picking technique had me brushing against knob and moving it. Used the o rings until I trained my hand better, then removed it

 

Best part- o-ring is like .10 and less than a minute to install/uninstall.

 

The felt washers used for strap buttons work perfect for this. I've done it in the past and had expected results.