Unlike English, Spanish is fairly easy to pronounce once you learn the rules. Vowels are tense and, with a few exceptions, their pronunciation never changes regardless of where or how in the word they appear. Consonants, also with a few important exceptions, mirror their English pronunciations for the most part. Following these three tips should have you sounding like a Spanish native in no time.

Choose the right audio program - You simply cannot learn Spanish without hearing it. Not only will a quality audio-based program like Pimsleur or Rosetta Stone help you learn the language quicker, they explicitly reinforce correct pronunciation right from the start. It's better to take the bull by the horns, so to speak, than try to correct poor pronunciation later in the learning process.
Listen to Spanish radio and television - Listen to how real Spanish speakers pronounce certain sounds you think you have trouble with. Mimic them aloud. Do you sound like they do? If so, move on to the next sound. If not, continue listening and repeating until your pronunciation sounds as close to theirs as possible. It should only take a few weeks of doing this perhaps 15 or 20 minutes per day to dramatically improve your Spanish pronunciation.
Read aloud - When you read a Spanish book or newspaper silently, you will tend to fall back into the habit of association Spanish spelling with its English pronunciation. This is counterproductive. Instead, speak the Spanish pronunciations aloud as you read. It will help reinforce the Spanish way of pronouncing words in your brain. It goes without saying you probably shouldn't take this advice in public, though!
As always, practice makes perfect when it comes to improving your Spanish pronunciation. Just 15 or 20 minutes per day can make a significant distance in how authentic your accent sounds.

Want to learn to speak Spanish but don't know which program is best? Check out my Spanish Pimsleur and Rosetta Stone Spanish reviews to make an informed decision. http://learnspanishonyourown.com/rosetta_stone_spanish_review.htm