Locate the key which is sticking and press is down along with the key on either side of the sticking key. Carefully insert the tip of a flathead screwdriver in the middle of the keys while holding them down. Make sure that you do not apply too much pressure or else you will end up damaging the keys permanently.
Piano keys are accustomed to movement in only one direction, especially if no one else but you play the piano. Slightly move the screwdriver’s tip in between the keys to alter their usual movement. Remove the tip of the screwdriver. This method will also help you get rid of small objects or debris logged underneath or around the sticking keys.
Check and see if the problematic key is still by pressing it down. If that is the case, repeat the screwdriver method at least a couple more times. If the key is still sticking, the cause of the problem is probably moisture around the key.
Use a business card folded in half to collect and thus remove the moisture or stickiness. Insert the business card in between the problematic key and the piano’s wooden edge. Push the card a little so that a small part of the card is under the piano’ front edge and run the card back and forth for a number of times. Wiggle the card as you run it back and forth to make the moisture stick to the card.
If none of these techniques works, you may have to call a piano repairman as a last resort.