I make very few trades in my leagues... just like an NHL GM, I make one maybe two trades per year. Very, very shrewd.

My #1 advice is use the following line:
I am interested in ______. What would you want for him?

The response will tell you whether you have any chance to pull off a trade with the manager of whether he is unreasonable.
It's a very quick, easy way to assess a prospective trading-GM.
Also, and this is critical... if you want to make a trade with another team to get his player... you have to let him pick a player from your team.

The biggest mistake I see is people straight-up making a proposal:
I'll give you Filppula & Simmonds for Scott Hartnell.
No, no, no. That's already being bossy and telling the guy what you think is fair.
Nobody is going to take your first offer.
AND even if it IS fair, they will think it's a low-ball offer.
So, this tact damages your own goods or sours the other guy.

I do realize some people use the Yahoo! trade function where you have to click players - but if that's the case, I just do this:
Seguin
for
Lebda

And I include a note: "This isn't a real trade offer... but I am interested in Seguin. Do I have anybody that interests you? If yes, send me back an offer".

The guy will either reply with a trade offer or just say NO.

Easy peasy.