If you have been speaking Chinese (or trying to speak Chinese) for a long time with sloppy tones, it will require extraordinary effort to improve. After you have mispronounced a word 50, 100, or 1,000 times, it is very hard to override those habits and relearn proper pronunciation. It is much harder to fix sloppy/toneless/inaccurate Chinese than it is to learn it correctly the first time around. Learning it properly the first time around requires that you have a teacher who knows how to teach tones, and that you have the patience to stick with it. Not all of us are that lucky.
Let’s face it, most foreigners don’t even pronounce nǐhǎo correctly. One must, of course, start with reading pinyin correctly, both the sound and the tone, for syllables in isolation. After you have learned to pronounce one syllable terms correctly, then you are ready to move on to two syllable words (combos). You will need to practice all the possible tone combinations: first:first, first:second, first:third, first:fourth, first:neutral, second:first, second:second, and so on. You can make up your own list from words you already know, or from words you find in the Cpod dialogs. Practicing with individual words is only the first step in learning to speak whole sentences or phrases correctly, but it is, of course, a necessary precondition for speaking well. I think a good teacher/tutor would be essential to learn to pronounce the two syllable words correctly.
When you can do that, go to the newbie, elementary, or per-intermediate lessons on Cpod. Go to the dialog or expansion sentences. Listen to the native speaker say the line, then you do your best to imitate it, reading from the pinyin version of the sentence. Read the pinyin version so the tones are all there for you. Listen to the native speaker repeatedly, and read the sentences over and over until your pronunciation, tones, and rhythm are very close to that of the native speaker model. When repeatedly reading these sentences, your goal is to speak accurately, only increasing your speed as you become more adept at mimicking the tonal contours of Chinese sentences. Reading dialogs and passages from Chinese characters should be undertaken only after one can read pinyin very, very well.
This approach works best if you have a good speaker of Chinese around to let you know how you are doing. In many cases, a good non-native speaker of Chinese will do this better than an untrained native speaker (with whatever local accent they may have) or an experienced, but burned out (native speaker) teacher.
To thejco: you also must be merciless with yourself! When speaking Chinese, if you don’t know the tone of a word: don’t say it. You are only reinforcing a mistake if you do.
To zaphekiah: It appears you have been poorly taught. Find a real teacher.
Nabeshima120: He makes a good point about listening closely. “If you can’t hear it, how do you expect to (re)produce it?”
I hope you find these suggestions helpful.