I am thinking the same thing recently.

Firstly, sentence stress is different from word stress. I think the principle rule is stressing what you want to express and what you want to deliver. Stress the word that you would like to emphasize. Most of time, they are content words having the actual meanings like noun, verb, adj,and adv.

When stressing a word, make it clearer, longer and a little bit more high pitch.

For the prep, pre-noun and be verb, just quick and soft.

Intonation is another story which I haven't done enough research on it. My feeling is, if just saying usual something, using low pitch; if you want to express your emotions like doubt, surprise......, use the the high pitch.

These are what I learnt recently during my reading. A lot of times, my muscles are still not strong enough to speak the non-stressed words quickly and softly. So it is easier to write than read :).

Practice makes perfect. Reading more, we are definitely getting better on both.

Just my two cents.