Listen to the tone and tempo of particular dialects. Picture the words as would sound coming out of your mouth. Saying the words convincingly is more important that properly pronouncing the words.
Use the influence of stronger dialects of a language to develop your accent. Many recorded dialect-coaching tapes use the mildest form of a particular accent in the hopes of making it more understandable. Listen to recordings of the thickest dialects of a language if possible. Work on learning the thick dialect. Mastering these will allow you texture your performance by grading down to a more mild dialect. Dialects tend to erode over the course of a performance, even among the most talented performers. If the dialect is strong to begin with it, it will remain fairly strong to the average ear throughout the performance.
Learn the slang and grammar unique to a particular dialect. Focus on the vocabulary of a region rather than the accent. Listen to the dialects of the people from the same region as your character.
Consider the age, class and upbringing of your character and how his particular life experiences might influence his speech. Focus more on developing the character of the role rather than using an accent or dialect alone to create the mood.
Do vocal exercises to develop a strong, free voice, for example: Breathe in through the diaphragm, inhaling and exhaling through the nose, sit in a chair and pant, without inhaling or take deep breaths and exhale slowly.
Focus on placing your tongue in the proper position when making the sounds of the new dialect. Let the tongue fall where it does when normally speaking then place it in the proper position to create the desired sound.
Take a class in voice preparation and accent learning. Find one of these classes by searching online or ask an acting school instructor if there are any recommended schools. Study the accents and dialects of the language you are trying to learn, as taught by the instructor.