I have had lots of trouble establishing sweet clover stands over the last 6 years.
I was able to get a great catch on some Acres that were in rotation for cover crop followed by Vegetable crops. Last year I let the Sweet Clover completely mature and then did a fall burndown to take care of some perennial weeds.

I was in the field yesterday and was ecstatic with the mulch created and the fact that I can count on over 100 Lbs of Nitrogen Credit.
I just need to get the recipe to do it every year.
Check out how it turned out in this video.