Getting the group together to talk about the game you want to play. This is important because everyone usually wants something different but if people are up front with what kind of game they are looking for or understand what kind of gaming they will be playing in then people will opt out early and you tend not to have players drift off without explaining why, it is often because they aren’t getting the play experience they want out of the game and don’t want to say they don’t like your game.
With 4E I’ve also found it useful if you have 4 or more players to try to have all four roles present at the table as well. Some ways to encourage that is to provide a bonus feat to the party if they can muster one of each role (the tribal feats from Primal Power work well even if they need re-skinning work well for this and can help to tie the group together). If you have a group that is a little more open to randomness then they could roll for initiative and the chance to choose a role in the party.
I’ve also always had good luck when a DM has provided the players with information about the world and has been willing to work with the players to fit in their own ideas. If somebody wants to play something that seems “wrong” for the game the DM & player have this chance/time to figure out and adapt or re-skinned to make it fit in. Sometimes that is as easy as saying they come from a different part of the world or escaped from some mad wizards laboratory. These have the hidden benefit of building PCs with instant plot hooks for the DM.
The last and possibly most helpful thing I have seen is to have the players create their party together. Make the first run a different kind of game than regular D&D. Build the party and bind them together in the past or with mutual points of contact or history. Providing small bonuses for linking the PCs together a trinket that will grant a one time bonus to a saving throw or the like. Depending on the breadth of PCs and the campaign world these links become very helpful when binding the group into the needed cohesion without feeling artificially forced, or I’m working with you because I have to but once we save the world I’m done with you kind of frictional parties.
Hopefully you find these ideas helpful in your game planning even if you are involved in a current campaign stopping for a little bit and regrouping with the players and DM might reinvigorate your group and encourage new growth in the campaign.