A couple points to mention when trying to convince someone that a trade is good.
1) Back your points up with statistics: What did the player do last year? What have they proven to do in the past couple of years? If they don't have a long enough track record, what did they do in the minor leagues? Simply saying "Yo, this guy is awesome!" doesn't carry enough weight behind it.
2) Position scarcity: If you are trading away someone that is above and beyond a strong figure at his position, then that should an asset in of itself. Troy Tulowitzki at shortstop comes to mind, while third base, second base, and even closer have proven in the past to be difficult to fill with confidence.
3) Team, ballpark, and opponent value: If the players look equal regarding statistics and position scarcity, point out where the player is located. If he's got great support from the lineup (Red Sox, Yankees, Phillies, etc) then he's more valuable than someone who plays for a bottom dweller (Pirates, Astros, etc). The ballpark has a lot to do with power or pitching potential, while opponents (especially for pitchers) could mean more wins or strikeout possibilities.
4) The other team's needs: Perhaps the most important aspect of a good trade is trying to help your team out while concurrently assisting the other team. The other team has to find a need in what you're offering, so whether it's position, youth, or keepability you need to make sure your trade is worthwhile for the other person for the upcoming season. Don't offer an outfielder if they've got plenty of them. Don't offer marginal players for a stud if the other manager doesn't have room for those marginal players.
Ultimately, you need to find a reason(s) for the other manager to accept your player onto their team. If you can't find an argument as to why they should bother, then they probably won't. Good luck in your upcoming season and your trading adventures.