The correct way to pronounce Porsche is PORSH-uh, with two syllables, rather than simply PORSH with one. Although the "silent e" in German can be emphasized or left out depending on the regional dialect, Porsche is a family name and, therefore, does not vary.
The difficult thing for many native English speakers is that the final syllable "uh" is soft and more similar to a voiced exhale than it is a syllable on its own. In addition, the actual German pronunciation of the "sh" sound in Porsche includes "s" and "ch" simultaneously, a combination that is not natural to the English language.
Like Porsche, other commonly mispronounced brand names include Nike, pronounced NIY-key rather than NYKE; Givenchy, which is pronounced Jhee-von-shee instead of Geh-VIN-shee; and Adidas, which is actually AH-dee-das with emphasis on the first syllable instead of ah-DEE-das.