Pitching is not a one time activity - success comes with repetition.

So each mound outing should end with a recovery program to prepare you for next time.

Younger players are more resilient and may not need to be as concerned about following these steps - but the sooner every pitcher gets into these good habits, the easier it will be to sustain them for a lifetime.
1 Pitch Release 
The most important recovery step may be while you're still on the mound. It takes every muscle group in your body to accelerate your arm and hand to the 70-80 mph plus of a pitch. But deceleration too often happens far quicker and with far fewer muscles - connected to elbow and shoulder only. The result can be a bullwhipping action that can severely damage your arm.

So, make sure your follow-through brings your pitching arm across your body to make contact. In other words, use your body to brake your arm action so your muscles take less of the strain.
2 Wind Sprints 
The second phase of pitching recovery should happen right after you leave the mound. The number one cause of muscle soreness has been traditionally blamed on a build-up of lactic acid. While that is no longer a widely held believe, you do want recovery to include demands on your full energy system, not merely drawing from the lactate. So run sideline wind sprints. Hard breathing will pump oxygen through the blood stream and that will reduce lactic acid concerns.

3 Resistance Flexing 
Done with the help of another player - to stretch out leg and arm joints and muscles so they don't tighten up. (If the pitcher is moved to another defensive position after leaving the mound, which happens often in youth leagues, then do the wind sprints immediately after the end of the inning and the flexing at the end of the game.)

4 Ice, if necessary  
If bruising or muscle pull persists, then ice. A bag of frozen peas may be good for small areas and readily available at any grocery store. Put a thin dishtowel on your arm then tape a small bag of frozen peas around it. Peas are great because they're small so will shape to your arm, and should stay cold long enough. (Remember to put a "DO NOT EAT" label on the peas so you can repeatedly refreeze and reuse the same bag. And if you need to ice biceps, triceps, deltoids and pectorals, then divide a bigger bag of peas into small ZipLoc freezer bags and tape them on.

If you want a more professional approach to icing use a shoulder-arm wrap. Icing should be 15-20 minutes max., just long enough to stop the microbleeding in the muscle tissue (that's all a muscle strain is).
5 Jog or Walk 
Especially if you've iced down to stop muscle bleeding, you'll need to jog to restore the core temperature in your body to normal. A brisk walk or short jog is all you need - maybe two laps of the field. (Do the wind sprints first.)

6 Feed Your Muscles 
Feed Your Muscles - The final phase after strenuous exercise (and an outting on the mound should be all of that) is to replenish the energy stores exhausted by the effort. Current wisdom is a good protein/carbo meal about 1-2 hours after the game. As mentioned elsewhere on WebBall, our top food recommendation is a banana - for the high potassium content.