One thing I would recommend doing early on in learning a language is making a systematic comparison of the sounds of your L1 and your TL. 1. Sounds that are similar in the two languages but differ slightly. What is the exact difference in terms of sound and mouth movement? Work on hearing the difference and on producing it. 2. Sounds that exist in your TL but not in your L1. First learn to hear them (which can be a problem sometimes), then the correct way to produce them. 3. Distinctions that exist in your TL but not in your L1 (e.g. in some languages, aspiration is phonemic). 4. Sounds that exist in your L1 but not in your TL. Avoid "subbing" them in for an analogous TL sound. 5. Sounds that are exactly the same between TL and L1 (may not be that numerous, depending on the language pair). 6. Learn what the typical "tendencies" of your fellow L1 speakers are when learning your TL, and avoid those (e.g. native Anglophones tend to put diphthonged vowels all over the place). In addition to this "atomic" step, there are also the word (stress, tones, contractions) and phrase (intonation, "linking" between words) levels on which you can make comparisons. Some of this stuff may be less well-studied or well-codified than phoneme-level and thus harder to attack systematically, but it may be worth trying as well. For me this sort of analysis helps in several ways: first, the "typical" pronunciation mistakes become much more obvious and thus easier to avoid; second, if I am told that my pronunciation is off in some way, I now have a base of knowledge from which I can work; and third, a purely psychological advantage, it makes the task look more manageable. When talking to fellow learners of French, it kind of surprised me that very few of them had ever done this kind of systematic inventory and most just seemed to sort of play it all by ear and imitation. Maybe this analytic mindset is not natural to some people, or maybe it just bores them. I know that some are not too fond of a "comparative" approach to language learning in general, whether for pronunciation or for any other purpose. But I think that so far it has aided me a lot in my efforts for a good pronunciation.
On systematic phonological comparison... FSI Basic Spanish lessons one and two do a great job with "minimal pairs" and comparing how various sounds are different between Spanish and English. FSI Programmatic Spanish also has a good bit of phonology (pronunciation) work in the first part of the first several units. FSI French Phonology is helpful for noticing and practicing the differences in sounds between French and English.
(Please ignore any erroneous capital letters -- I'm on an Italian computer that keeps "autocorrecting" various English words as though they're part of a song-title or something, and I'm sick of fighting against it.) I've always been of the opinion that you have to learn to produce first, and that only then will your brain learn to hear the difference. Obviously infants don't work this way, but I don't believe that adults work the same way. There is a theory that says that Language is a reflexive act, and that we perceive phonemes by reconstructing how we would pronounce them ourselves. If such a theory is correct, it follows that our brains' natural tendency is to perceive our "nearest equivalent" and ignore the difference (which is how we deal with other people's accents, after all). Training the mouth in the physical movement and being meticulous about pronouncing foreign phonemes should, in theory, teach your brain that these mouth shapes are meaningful, and therefore Worth perceiving. Completely agree. Awareness of your most likely mistakes helps avoid them. The one thing I'd add is a word of caution about phonemes vs allophones. (For the benefit of those not familiar with the terms... A "phoneme" is a meaningful sound -- essentially the equivalent of a "letter" in speech. In handwriting, a single letter can take many forms, depending on the letters before and after -- similarly, a phoneme can take on many forms depending on the phonemes before and after. These forms are called "allophones" of the phoneme.) "Learning pronunciation" usually really means "Learning accent", which means Learning to pronounce all the allophones from day one -- this I think is a mistake. I prefer to reduce the difference between allophones to force my brain into recognising them as a single phoneme. I've met far too many learners who appear to have learned allophones as though they are two distinct phonemes, and therefore struggle to remember whether (for example) a given D in Peninsular Spanish is pronounced /d/ or /ð/. The most mind-boggling example of allophones, though, has to be Welsh, where the vowels have three allophones based on their position in the word: ante-stress, stressed, post-stressed. While my classmates were struggling with which A sound they needed, I ignored the difference, and just picked what seemed like the "central" point of the phoneme and used it in all positions. My teacher kept complimenting my pronunciation, so I must have been doing something right....
