Remove the typewriter's cover if the machine has one. Use a screwdriver to remove any screws, then lift the cover free. Remove the ink bobbins as well and set them aside.
Dampen cotton swabs with plain water and clean between the keys that are sticking. If the keys have not been cleaned in a long time, this may take a bit of time and pressure on the swabs.
Blow away loosened dust and grime with a can of compressed air.
Apply graphite lubricant to the sticking keys. Graphite lubricant is available in a small tube. Hold the tip close to the item you are lubricating and squeeze the tube to blow the graphite onto the keys. Use the can of compressed air to blow away excess graphite.
Press the keys to test them. Apply additional lubricant if the keys are still sticking.
Replace the ink bobbins and typewriter cover.