Typically in Mid-June I start looking for trades that could upgrade my keeper situation, without hurting my current team much. This usually comes from depth, I typically have extra OFs that I can 2-for-1 with pitchers or position players (or other OFs) without much of a drop-off on my current team.
Early July is my buy or sell date. I start looking at whether I'm a buyer or a seller, and to what degree. I only play in HTH nowadays, so I look at my chances of making the playoffs to help my decision and sometime in the next week or two put myself in 4 categories...
Strong buyer: I have a very good, deep team that has been playing well, is a basic lock for the playoffs and has a good chance to win it all. I won't mortgage the future under any circumstances, but I'll wound my future a little if I'm in this category.
Weak buyer: I am in good position to make the playoffs, and generally like my team but I'm not one of the top two teams. I won't hurt my future in this instance, but will focus on making trades from my depth to help somewhere I'm weak at. Basically I'll treat it as a complete re-draft league and not hurt my future, but at the same time not make much effort to help it.
Weak seller: Usually either fighting for my playoff life but not liking my team much, or on the outside looking in at the playoffs but close enough to make a shot. I won't give up on the season at this point by any means, but I'll try to make trades of my deep areas (again usually OFs) to help me in the future either with draft picks or upgrading keeepers. This is where I'm gonna fall this year in my main league. I figure if I have to catch lightning in a bottle to win it anyway, a few downgrades here and there aren't likely to make the difference. So I still have a shot at this year, but at least if nothing else I've helped myself down the road.
Strong seller: If I'm completely out of it July 1st is the time to sell. Get the picks/keepers before your competition of suck does the same. At this point I'd sell rights to my team name for a late-round future pick. The problem is that if I'm this far out of it usually I have injuries or horribly under performing players that are harder to trade for a positive next season. I'll usually start with lower-level package deals and continue to upgrade my players through trading away depth until I have something that might warrant a nice draft pick or that I can package for a big-time keeper.
That said, regardless of the situation I'm in, if I see a deal I consider incredible value I'll take it. If nothing else I can always use it for trade bait.