Keep your pants on -- no need to remove them. Grab a bar of soap.
Position one side of the soap against the teeth of your pants zipper and rub it up and down on the teeth to deposit bits of slick soap.
Start working your zipper to loosen it. Pull it up and down. Once it loosens, unzip and zip it several times to make sure it's good to go.
Squeeze a bit of hair conditioner on your fingers.
Slide your fingers up and down the zipper teeth and track to lube them.
Try the zipper. It may take a couple of tugs, but the conditioner will help it get moving again. Zip it up and down all the way a couple of times to make sure it won't give you any more trouble.
Dig in your bag for your trusty tube of lip balm -- flavored or not, no problem.
Pop off the cap and rub the balm up and down the zipper teeth.
Take hold of the zipper and give it a jerk to get it moving. If it doesn't move one way, try it the other way. Keep trying until it starts moving. Work it up and down a few times to lubricate the teeth.
In a pinch, grab a candle that's close by and rub the wax into the teeth of the zipper.
Stand over a trash can if you're worried about soap fragments dropping on the floor.