I haven't done much magic trading, but I have years of experience trading cards in other TCGs. The best tip I can give you is to always try to trade up. By this, I mean try to only make trades that will increase the overall value of your binder. This can be as simple as getting a $21 card for a $19 card, but it can also be more complex.
A key aspect of trading up is acquiring and providing play-sets of cards. Playsets are valuable because they give you a lot more bartering power in trades and tend to draw a higher trade value than the individual cards in the playset. Let's say that there is a card that individually sells for about $10, but is ALWAYS played by the playset in a top tier deck. Let's call this card "Bad Banana" Owning the full playset of Bad Banana and having it in your trade binder gives you two advantageous positions as a trader:
A player wants the full playset of Bad Banana. You are able to get more value out of the full set than just 4 x $10 because that player doesn't have to go to anyone else or go online and wait for those cards. You provided a service in finding those cards and putting together the full playset, so you can negotiate a small finder's fee into the trade. Usually a few dollars more than the individual values of the cards added up. In this case, I would value the set of Bad Banana at $44. Rarely have I ever seen a player pass up a playset of cards that they need because I valued the playset of cards slightly higher.
Player doesn't want the full playset, but understands that you may be hesitant to break the playset. Once again, this gives you more bargaining power, as you may be able to fetch a higher value for the card as compensation for breaking the valuable playset. This tends to happen more with rarer, or more valuable cards that are highly sought after by players.
On top of this is the obvious aspects like keeping up on the prices of cards that you are commonly seeing in play and in people's binders as well as working on trade etiquette. Be polite, ask questions, and don't be afraid to walk away from a trade if you feel like you're not getting what you want out of it.