Some of the elements necessary for great parody and satire are:
Passion. If you are going to write this type of comedy you need lots of passion for your subject. You can passionately love your subject or be just as passionate about how much you hate it. If you don't really care either way about the subject, you'll find that there's not a lot to sink your comedy teeth into.
Timing. If you are writing a parody of a commercial that is two and a half minutes long, make your sketch two and a half minutes long. Don't make it five or six minutes long. Watch some commercial parodies and you'll notice when the idea has worn out its welcome. That's the point at which the parody should naturally end. It's better to sacrifice some jokes than let the sketch go on too long.
Mirroring. You want to make your parody so close to the original that it's hard to tell them apart. In the early days on Saturday Night Live, a commercial parody generally followed the opening monologue's final line: “Stick around. We have a great show. We'll be right back!” The audience expected a commercial break; instead they got a parody.
However, the beginning of the parody seemed so real that they bought it hook, line, and sinker. They were misdirected and ultimately fooled. If you're writing a commercial parody, you want to mirror the original almost line for line, shot for shot. You really want to make the audience think it's the real deal. The more realistic it is, the more impact it will have.
To get a look at some of the best commercial parodies ever made, check out the DVD Saturday Night Live: The Best of Commercial Parodies. Watch the ones that are dead on and the ones that are a little off the mark, and use them as inspiration to tackle ads that are currently in the public eye.