It is definitely important to know your league. But the first thing you want to do at this point is evaluate, or perhaps re-evaluate, your team.
I generally break my roster down into three classes of players:
1) Definite freezes. These are the no-brainers, and the guys who are going to form the core of your value going into your auction.
2) Definite drops. These are guys who are no-brainers the other way and you're not even thinking about keeping.
3) Borderline guys. This camp can be broken down into more than a few types of players. Some of them are veterans who have been around for a while. Their status won't change much between now and Opening Day, so it's just a matter of deciding if they'll fit on your roster. Others are rookies or players whose roles are undefined, or players returning from an injury. It's impossible to say now whether or not these players will be freezes, since Spring Training tends to sort these jobs out.
Category #1 is the easiest one to define. There aren't going to be too many guys in this group that you have to give much thought to, unless someone makes an enticing trade offer.
One of the tricks, though, is identifying which guys you might think are definite drops that your opponent thinks are borderline or even definite freezes. This is how trades are created.
Going back to John's example from Wednesday, I tend to be conservative with my freezes. This stems mostly from the fact that I'm confident in my ability to buy a good team on Auction Day, or replace some of my borderline guys with better value. However, not all owners have the same level of confidence in their skills that I do.
My favorite trades are finding guys I'm lukewarm on and trading them to another team. Ideally, I like to get a player I'm going to keep. But I'll even make the trade for a player I know I'll throw back for a few reasons:
First, the player I acquire will automatically generate interest from other owners, even if he's not much of a freeze. Psychologically, other owners think that a player worth trading for is a player who might be worth freezing.
More importantly, though, I'm doing two things by moving a non-freeze off my team. I'm clogging up another team with what I feel is an overpriced player, but I'm also opening up my roster for a more talented or equally talented but cheaper player later on.
Finally, adding a player who is overpriced to another owner's roster will actually reduce inflation. This is generally a good thing (though not universally, which is what some touts mistakenly teach) and will probably help me out in the auction by reducing the price of my team by a couple of dollars.
By all means don't limit yourself to these types of trades. But look for the opportunity to make these smaller deals, and subtly add value to your team by subtracing value from one of your opponents. Trust me, it's not that hard to do.