Everyone wants a better serve because a better serve almost certainly means that your game will rise to a higher level. The serve is the single most important shot in the game.
The serve can only improve in 4 ways
Luckily there are two easy to learn techniques that if mastered, will give you all four of these desired outcomes.
1. The front foot
The front foot for a right-handed player is the left foot, and it's the right foot for a left-handed player.
The function of the front foot is to become the "bridge" for weight transfer from the back to the front. Think of the weight transfer during the serve as an action involving three points. (1) Weight on the back leg/foot (2) weight transitioning to the front foot, and (3) the finish position after contact, which is forward and inside the court.
The important middle phase, phase 2, between the back leg/foot position and landing forward into the court after completing the serve is your front foot. It acts as a "bridge", staying stable as the weight transfers from behind to the forward position, and it supplies energy to thrust the body up and forward to hit the ball.
The front foot also performs several other important jobs. The timing for your serve will come from your front foot and you should place your awareness there during the service motion. If you develop feel out of the toe of the front foot you will be able to time the ball perfectly every time because toe awareness allows you to fine tune the serve and achieve that effortless "pop" on the ball that all advanced players have.
Lastly, the front foot creates a powerful interaction with the ground similar to the final foot plant of a high jumper, building tremendous ground forces that transfer through the body and into the arm and wrist and eventually the ball. Power for the serve comes out of your front foot.
The final "must have" service technique is...
2. The Wrist
If you can maintain a loose wrist at the end of the service action you can use it to direct the ball (left or right) and achieve depth accuracy (deep or short) and the degree of spin you desire ( excessive spin or relatively flat).
Here are the things you need to be aware of for accuracy, direction and spin.
You can achieve a deeper serve by hitting the ball later in the contact zone. Hitting late means the racquet face will be open on contact, which will send the ball longer.
By creating early wrist you achieve the opposite, this time the racquet face strikes the ball early, sending the ball short.
Direction means degrees of the left to right axis. By bringing the wrist through to the left or right you can archive simple direction variations. Practice enough and you can soon target the lines of the service box easily.
Make sure there is no racquet-head contamination as you follow-through. Avoid squeezing the grip prematurely, simply allow the weight of the racquet head to dominate the follow through completely.
The methods spin is taught on the serve has been far to complicated. To achieve spin on the serve simply finish your racquet-head above the contact point. Less spin means finishing slightly above contact. Increasing spin means finishing well above contact. It's that simple.
You can finish above contact either by means of the elbow (be careful to never extend the elbow completely straight as this creates tennis elbow), the wrist, the legs or a combination of all these methods.
With spin you can achieve increased consistency and variety. When you focus on spin, as with a 2nd serve, become more aware of what the ball is doing in the air, meaning the flight of the ball and where the 2nd bounce will finish. The 2nd bounce tells you where the returner will have to position themselves to hit the ball and your preferred 2nd bounce target will be mostly outside the doubles sidelines and/or high and deep.
So you can see that by developing the front foot and the wrist you can open up a new world of serve potential.