It's really hard to do, but you need to find a team with really high interest and it would help if they have cap and roster space... also it works if you agree to take a highly-paid player that sucks off of their hands
most of the good draft picks (bottom teams who have a high chance of getting the top picks) are really hard to trade for. You'd probably need a 70-85 rating guy to trade for a 1st round pick from the worst team in the league. You would probably be better off trading for the rookie you want when the draft is over.
the computer was being mad stubborn and wouldn't take any of my trades even though clearly it showed that they were getting the better deal, including draft picks.
That's happened with me a lot too. I've been trying to get LeBron James, but his and the guy I'm trying to set up deals for a trade with, salaries don't match.