I have done pretty well in my leagues, and as a result some owners gun for me specifically in the auctions or drafts. Even to the point of bidding for players I nominate just because they think if I nominated the guy, I want him and he must be worth it. Can really make some owners waste cap room and precious roster spots near the end of the auction when they do this.
Anyway, this kind of reaction to my showing interest in a player impacts my trying to make these kind of trades. If I'm filling an obvious hole on my team I can be pretty open about a trade. But if I'm pursuing a sleeper or someone I think is undervalued, then the fact I approached them without with an interest in Player X without a glaring need often seems to increase their opinion of him and make it less likely they'll trade him.
So for this kind of thing I try to let the other owner be the one to come up with the basic trade, or at least to set some parameters for me where I'm choosing from what he's given me rather than obviously pursuing someone specific. Having a valid basis for the trade is first. "I've got a little extra RB depth and wouldn't mind more WRs on my bench, and I see you have WR depth but could use RB. Which of my players might interest you, and how do you feel about your own in terms of which you're most and least willing to trade?" I may include mentioning some of my views on my own players, or if not I'm ready to come back with that to help things along if his initial response doesn't give much. "I value my player X over Y, and Z I like quite a bit more than either so I probably would need more for him." Another approach could be, "I've got your backup RB and he's worth more to you than he is to me since if your starter goes down you have more need for him then than I would. So thought maybe there was some kind of win-win deal we could find. I might be looking for a WR and was wondering how you'd order your backups etc etc."
Hopefully that gets a discussion going where he gives me an idea how he values his players. Now I can craft an offer that includes the player I desired, and hopefully is a win for him but didn't let on that I wanted that specific player. Instead I'm just responding to what he gave me to go by.
I don't just use this approach when I'm after a specific player either. I find getting someone to think about how he values his own players relative to each other and share it, and my doing the same, really helps us come up with a trade both owners are happy with. So this kind of discussion isn't unusual for me to use in any trade.
Another angle you could use would be swapping two players you think are comparable and throwing in a couple of others to even it out, where the player you wanted is one you get in the exchange. But of course that only works if there's such a trade that you think is even enough you can tack it on to.
And another approach would be to start out by openly pursuing some other player at the position, one I might not even really want. For those owners who I know most let my interest increase their opinion, they focus on the first player we discuss. When we're not able to consummate a trade for the first player, they feel satisfied that their player they are keeping is worth more than they thought. Now I can bring up the second player as someone I'm less interested in, but if we can't do a trade for the first guy I "really wanted", I might settle for him instead. They feel like they came out even more a winner because they got me to settle for someone less.
