What can I do to increase my VO2 Max?

My times really aren't that bad for my sprints of 50 yards, or even 75. From a push start, I can go 27-28 for a 50, which I assume is respectable since I just started swimming again earlier this year. My problem is that I have a hard time keeping a 1:15-1:20 100x7 with 10 to 15 seconds rest. The first 3 or 4 is no problem at all, but after that I hit a wall and am sucking air big time. Also, I am really gassed if I do flip turns. If I slow down to 1:30-1:35, I can do flip turns and go on without being to winded by the end of the set. My question is this: What should I be focusing on to increase my VO2 Max? Sorry if my question is to general. Hook'em Blue