Begin by educating your child, even when he is very young. Explain what medication is and how it helps him get healthy. Be clear that taking the medicine is nonnegotiable, and that “we all do it.”
Give him as much of a choice as you can about the process—who gives the medicine to him, when to take it (within reason), and what drinks he can choose from afterward.
If using a medicine syringe to squirt the liquid into his mouth (not for subcutaneous injection), aim for the back of the cheek. Harder to spit out, easier to slide down.
Let him give you your medicine whenever you have to take it—this will show him that medicine is not so bad after all. Thank him profusely for helping with such an important chore.