Regular push-ups are so easy that even a baby can do them. Time to grow up.
Push-up Variation 1: Ring Push-ups
Perform push-ups on gymnastic rings or homemade suspension training system. The rings will make you shake like a little baby at first but they will increase the chest activation during the movement and will make your shoulders stronger.
Just find some old chains and wrap them around you and you are ready to go. Keep in mind that when you are at the bottom the chain will touch the floor and the difficulty of the movement will be diminished. This is not a problem if your trying to hit triceps because at the top the resistance will increase once the chains are lifted from the floor. However if you are doing push-up for chest just elevate yourself using a pair parallettes, push-up boards, books, rings...etc.
Push-up Variation 3: Doing Push-ups With Chains On Rings
Doing chain push-ups on rings. This is just combination of the two exercise above.
Push-up Variation 4: Single Hand Push-ups
You can greatly increase the difficulty of the exercise by performing the regular push-up using only one hand. Keep in mind that this will place most of the stress on your shoulders and triceps while leaving less for the chest because in order to balance you will have to keep your hand pretty close to the side.
Push-up Variation 5: Clapping push-ups.
Adding a clap between your push-ups will make the regular variation much harder and will increase your dynamic strength. Keep in mind that you need a solid base prior to trying this variation otherwise your are looking for potential damage to your joints.
