Some great advice has been shared here by other respectful members. So I wanted to add some additional points...
It's important to remember what the real purpose of your subject line is. It is to make your potential JV partner interested and curious enough to open your email and start reading it.
Many people make the mistake of trying to "sell" with their subject line. It's when your subject line will just turn off people because it looks like a sales pitch... a "too early" sales pitch.
But here's a very powerful tip to at least double the responses you receive:
Don't just send a JV proposal as your first email to them.
Build a relationship with them first.
A great technique I've used myself several times and have found very effective, is starting your conversation with them about one of their articles you've read or one of their product that you have used and found helpful. The key is to start talking about THEM first.
Give genuine compliments about their products and websites.
Show interest about them.
And it's very important that you do this all genuinely, because if you fake it, they will know the first moment and will lose all respect for you. So you'll lose them as a potential and valuable JV partner forever.
So first build a relationship with them and then approach them with your offer. It will increase your success dramatically. I can't emphasize it enough how important it is.
By the way, I read an awesome course about how to have a super success with JVs a while ago and truly loved it. I had read many other courses before but this one was really amazing. The author himself is also one of the most successful JV marketers and brokers.
If you're really serious about having big success with JVs, I highly recomend this course. It's called Instant JV Success System by Chris Rempel.
Wish you good luck,
Ladan
P.S. I'm not affiliated with the author of this course and I won't get paid if you order it. I'm just sharing it with you as a friendly recommendation.