The tips I'm going to give you aren't exclusive to Spanish, but I think you'll find them helpful. I have three of them for you:
1: Take advantage of dictionaries with audio pronunciation.
Every time I read, I almost always come across a word whose pronunciation makes me curious. It isn't always a difficult word. Sometimes it's a word that I may have just heard pronounced differently by different people. This is true of both my native tongue -- English -- and others. Spanish rules of pronunciation are pretty consistent, so, oddly enough, I wind up doing this more for English than for Spanish, but if you're having trouble with a Spanish word, force yourself to take the time to do this. Some good resources include the following:
Each entry usually has several recordings from around the world and they almost always include at least one from Spain. Plus, if you actually take the time to set up an account (and it's super easy), you can actually download audio recordings.
Unlike Forvo, you will only find single word entries on this site and the only verbs you will find will be in their infinitive form. Neither will you be able to request a pronunciation of a specific word or phrase. You will also only find one set of pronunciations for each word, but that set includes a pronunciation from three different areas of the Spanish-speaking world — Mexico, Spain, and Argentina — for what I would guess is 99% of the words in its database. (In other words, a few words don't have pronunciations you can listen to, but just a very few, and they're usually long words that aren't that common.) Plus, you're given a choice of playback speeds — 100%, 50%, 25%. If you want to know what "purr" sounds like in Spanish with a playback speed of 25%, listen to this. You'll have to adjust the settings yourself using the dropdown box to the right of the word.
Tried to find a word that most would find difficult to pronounce, so please click on the word "Wiktionary" above to see what I came up with. Wiktionary also has a Spanish version, but its database of Spanish words on its English site should be sufficient for most. If not, the Spanish version of it is here. Bear in mind that it doesn't contain much audio (if any) and what it does have will likely be in English. (After all, its Spanish site is designed for Spanish speakers, not English speakers learning Spanish.)
Sometimes gives you the option to view entries from the Oxford Spanish Dictionary in addition to those from the PONS dictionary. It also gives you the option of listening to "European Spanish" or "Mexican Spanish," but both versions sound a bit computerized to me. For recordings that sound more authentic, use Forvo.
A dictionary with pronunciations. This site has a couple of neat features the others don't. For starters, it gives you details such as the number of translations found for each word and orders them by most to fewest. The word "claro," for example, has been translated as "clear" more often than "light." Without any context to guide you, you can assume that "claro" means the former and not the latter (unless you're in a conversation with someone in which case it likely means "of course" or "sure"). The other neat feature of dict.cc is that, when possible and appropriate, it provides an image of the word in the upper right corner. (Oddly enough there's an image for "perro" but not one for "gato.")
All of the resources listed above are online, free, and available for multiple languages.
2: Follow Sergio Romero's advice
In case you missed it, he recommended reading as much as possible and I think that is excellent advice. He suggests that this bit of advice is slightly counterintuitive, but it will seem more logical to you if you also read out loud. I recommend starting off with very easy books and progressing from there. If you go to Amazon.com, you can search for children's books by language and age level. Here's a search of children's books using the search term "car." I then filtered it to return just Spanish books for those aged "Baby-2":
Books in Spanish about cars for those aged Baby-2
Start off with the easiest and work yourself up the age levels. Many of the books will allow you to preview the book so that you can determine if it's the right level for you and whether or not it's a good investment of your money. As you do this, be sure to read out loud. As you conduct your searches, I think you'll discover that Amazon's search engine returns results in a somewhat mysterious way, but for the most part you should see books that start off at a basic level. If you are unsure of what age level the book is actually intended for, click on the thumbnail for the book and then scroll down to view the book's details which should include its intended age or age range.
I'm pretty sure you might be able to do similar searches via other sites such as Project Gutenberg and Open Library but few, if any, allow you to search for books like Amazon does. I know some Spanish sites contain a children's book section. I haven't reviewed them in a while, but if any are worth recommending, I'll add them to this post.
As for the ones you'll find at Amazon, some books even have audio, but be sure to listen to a sample of it first if available; it may not be of a quality, dialect, or style that appeals to you.
If your budget's too tight for the purchase of a book, pay a visit to your local library. I'd be surprised if it doesn't have books in languages other than English on its shelves. The United States might not be as multilingual as the European Union, but there's a reason we call this country a melting pot and your library shelves should be reflective of that. If not, contact your librarian to see how you can request books not currently carried.
3: Adopt tips and tricks used by those in industries that require flawless pronunciation of words
This one's going to sound a bit crazy, but it is a technique I learned while attending a basic journalism and broadcasting course. I graduated from the course many years ago, but I have often used this technique to improve the pronunciation of my own native tongue. Whenever I am preparing to deliver a message, whether it's an important message on a voicemail machine or a 5-minute speech, I read the message out loud with a pencil in my mouth. I am not kidding you. Granted, if it's something as long as a 5-minute speech, I usually only read parts of the speech out loud and usually just as a warm up, but I cannot emphasize how useful this is in limbering up the muscles of the mouth and helping you articulate your words more precisely.
I've lately begun using this method with Spanish. If I catch myself stumbling over a word while reading out loud in Spanish, I will stop, put the pencil in my mouth and repeat the word a few times. You think rolling your Rs is difficult? Just try it with a pencil in your mouth and you'll find that it takes "difficult" to a whole new level. Even so, when you remove the pencil from your mouth and try pronouncing the word again, you'll be amazed at how much easier it is to say that word.
I believe tongue twisters and varying the speed of your speech are also effective limbering exercises, and ones I've encountered in places outside of broadcasting (e.g., theatrical productions) but nothing I've tried thus far really surpasses the effectiveness of the pencil method.
I realize the coveted green checkmark has already been awarded and I'm certainly not trying to convince you to give it up for me. Many of the answers here were quite good. I just saw an opportunity to contribute something useful or helpful by sharing some of my own techniques. Usually my contributions are in the form of a question, so it was refreshing to see a question on a topic I felt I could be helpful with. I am by no means an expert on this topic and, in fact, stumbled upon this question simply because I was about to pose a question somewhat similar to this. I may still post it since the topic is a bit more specific and not likely to be answered if I merely tacked it on in a comment here. If I do, I'll post the link here in this answer.