Here's a quote from the thread titled "TOM : can you share more on how to improve pronunication" at
Apart from recommending http://www.antimoon.com/how/pronunc-how.htm
as a basic resource, I'll just paste my recent reply to Wingyellow (he also asked about our pronunciation software):
Here's how I learned English pronunciation:
1. Learned phonetic transcription and got a dictionary with transcription.
2. Imitated American versions of the sounds by listening to Shaggy Dog Stories, CNN, and my English teachers. When listening, I'd often try to pronounce some phrase exactly like the speaker.
3. Looked up the transcriptions for a LOT of words. Every time I looked up a word I also checked the transcription and tried to pronounce it.
4. The rest (word linking, sentence stress) came through listening. I picked it up naturally. Actually, I didn't even know the word "linking" until about 2 years ago.
In order to imitate native speakers' pronunciation successfully, you need a reasonably good ear, i.e. you need to be able to hear the difference between your version and the original (if there is a difference). I've always had a good ear, but many learners do not, as in the following typical situation:
Teacher: Okay, repeat after me -- BED.
Teacher: No, say BED
Teacher: No, that's not right.
Student: What do you mean? You wanted me to say BET and I said BET.
If you don't have a teacher who will point out such discrepancies, I've found many learners find it helpful if they can listen to their own voice alongside the proper recording.
PerfectPronunciation is something we developed to make it easier for learners to do step 3, i.e. learn the pronunciations of English words and keep them in memory. The program also allows you to record your own voice, compare it with the proper pronunciation, record it again, listen to it again, and so on ad nauseam. I think it's helpful, because it lets you listen to good pronunciations of 500 English words as many times as you want, compare your voice with the original, and also drills you every day (so it forces you to learn regularly and chooses the questions according to their difficulty).
I want to stress that obviously I did not get my accent thanks to PerfectPronunciation (which we developed this year). I didn't have to record and listen to my voice at all; I can just hear it when there's a difference between my pronunciation and the source, and with patience I can get it right eventually. Sometimes it takes weeks to arrive at the right version, and even more time to use it consistently.
It's important to listen to a wide range of speakers within the accent you want to master (e.g American English). [Added Dec 23, 2003: Obviously, it also helps if they speak slowly and clearly. Movies are not a good idea for a beginner. Going to America and talking to people in the street is even worse.]
Sometimes, you practice one day without good results. After a few days, you go back to practicing, and you find you can produce a much better version now. Your brain and speech organs have gotten used to the new sound.
For points 2 and 4, it may be helpful to read (in books about phonetics or online resources) how the sounds are produced (whether you should round your lips, etc.) and read about linking and sentence stress, looking at examples.
I would add that if you're reasonably good at imitating other people in your own language, you shouldn't have too many problems learning English pronunciation.
To answer your question: My "conversational" pronunciation is usually worse (i.e. less American), because, being in Poland, I practically don't speak English. It takes me a couple days of speaking English to get to my best accent.
Also, I've found that talking to Britons prevents me from speaking American English really well. I tend to adopt the British accent when talking to them. It's still close enough that they take me for an American, though.