I've always have had a lot of trouble running games over the summer; either I've just wrapped up a long running campaign and I need a bit of a refractory period before the next one, or all my players have disappeared for the summer and no one is around to game.
I've tried a few things to try and over come the summer lull. It is a great opportunity for a new GM to step up and run the new regular game. A couple summers ago, one of my friends took over for the summer and ran a heavily homebrewed version of the Pokemon Tabletop rules. It was a fantastic game and really kept us all playing through the entire summer.
The other strategy we tried was running alternating one-shots or short, 2 to 4 session games. This was a great opportunity for us to try out strong, tiny game ideas. For example, the same woman who ran the Pokemon game ended up running a short Honor and Intrigue game which introduced a lot of my extended group to the Barbarians of the Lemuria system (a personal favorite). This strategy also let me run Dark Heresy for all these 40k neophytes, which was so much fun I had to blog about it.
This summer, I ended up starting a new weekly campaign which has been absolutely wonderful. The weeks running up to GenCon the players were able to cover a lot of ground, leaving us with a great cliff hanger before we all headed out to GenCon.
This coming Sunday my group is picking up deep within the Cloakwood Forest hunting down the lost Manse of a lost, legendary wizard. And a mine the Iron Throne is interested in...acquiring. While also saving Ankylosaur babies from Allosaur assaults!