Trades

In fantasy football there aren't that many ways to improve your team, especially when you lose a player because of injury or a player with a lot of expectations busts. The first way is through the draft. Every league has one draft at the start of the season where members systematically go through and pick players. Most drafts are snake drafts; meaning the person who has the first pick one round has the last pick the next round and so on. This evens out the draft a little bit so that having the first pick doesn't mean you automatically have the best team. Another common way to gain an advantage in fantasy football is to pick up players off the waiver wire. The best situation to pick up players is when a key player is injured (specifically running backs) because their back ups will have to pick up some of the slack. The problem with the waiver wire is that most leagues have some sort of system to allow lower ranked teams to have higher priority, so if you have a good start to the season and you start slumping it may be hard to pick up players.
By far the most interesting method of improvement is trades. Trades are fairly self-explanitory, two people exchange players. In most cases, trades are made when one member wants to improve a certain area of their team and are willing to give up something the other member wants in return. In my experience, trades are usually a toss up and you can't see whether someone has gotten the better of a deal right away, you have to wait a few weeks to see how they perform. One of the worst feelings in fantasy is when you trade away a solid player, and the player you get goes out for the season with a torn ACL. Come on Keenan Allen.