This should be pretty obvious, but I watch the same teams year after year agree to two for one or three for two trades. If you get more players than you traded, 19 times out of 20, you are a sucker. Understand? If you aren't willing to take this step for yourself, do it for your league...making these trades just encourages the guy who sends lousy trade offers every week to keep doing it (oh wait that's me...nevermind). Actually, the leagues I'm in are very competitive and it's been fun watching owners wise up to bad trade offers. You only see one or so a year that make you cringe.
In theory, you can make 2 for 1 trades that are balanced, but what owners perceive as "balanced" is often skewed because they forget to, or incorrectly, value the open roster spot. You see, the owner getting the "one" player in the deal is getting much more than that, they are getting, at a minimum, the best player on the waiver wire in addition. But that just scratches the surface. Open roster spots are valuable, owners can be aggressive picking up speculative save opportunities or a hitter on a hot streak just to see if he turns into this years Jose Bautista or John Axford. Think about it, how many times have you struggled with whether to drop a guy who is struggling for a hot week sitting on the waiver wire...you wait b/c you don't want to give up on a guy, and before you know it the hot waiver wire pickup is gone.
In other words, an open roster spot gives you the opportunity to "try out" guys. If you like to pick up SP based on matchups it's great for that too. Roster spots are valuable, and the best owners know that even if the waiver wire looks bleak this week, there undoubtedly will be someone they want...soon. On countless occassions I've picked up someone that is comparable if not better than one of the two guys I just traded. It's a great feeling. There are exceptions...particularly later in the year if you find your team way back in saves, steals, starts, etc...it is conceivable you'd trade one pretty good starter, for example, for two closers or vice versa. It doesn't happen often, and if you aren't sure don't do it.
Want to stop being a fantasy loser? Get quality over quantity in your trades and don't receive more players than you trade away.