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If you didn't get around to doing all that winter sealing to stop the heat getting out, it's still important to do it. You don't want the cool air from the AC escaping anymore than you did the heat from your home in winter. Block all those gaps around window frames especially.
Check the temperatures that you can use caulk in before starting. You may need to do it on one of the cooler days.
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The best thing I ever did was use caulk in the cupboard under both my kitchen and bathroom sinks. They were very, very draughty. There were several cracks in the walls behind them. I also used that foam stuff that hardens in all the gaps that were left from cutting holes for the pipes in the cupboard's back and bottom panels. It made a big difference.
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I'm feeling smug because we insulated every nook and cranny we could last winter, including adding more to the attic. I feel that's been the biggest saver of all because you could touch your hand to the ceiling in a room and feel the heat from what was building up in there in the summer. That heat was seeping into the rooms from above. The same thing happened with the cold in winter. Now you don't feel any noticeable difference at all, so the insulation's doing its job.
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I put up thick plastic panels on my windows in the winter, on the inside. I'm talking about transparent plastic that's solid and a half inch thick. This year I'm not going to take them down as normal. I'm going to see how much that helps on the AC too. I may never need to wash the inside of my windows again LOL. ;)
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I devoted myself to going without the AC this summer. This article made sense to me: http://billeater.com/tips/7-ways-beat-summer-heat-without-wasting-money-ac.
I'm going to see if I can make it the whole summer. They say that if AC was suddenly banned, Florida and Arizona would empty out. People can only live in those really hot places because they have AC. Of course, I have a cool breeze off the Atlantic here, so I've got a better chance than many to make it through the summer.
Dupont gives advice on that.
http://www2.dupont.com/Caulk/en_US/applications/caulking_windows.html
This Energy Star guide on sealing and insulating tells you how to find air leaks. It explains how to fix the big ones like attics too. Might as well do it all at once! :) You can get a printed copy from their Publications page too if you prefer.
http://www.energystar.gov/ia/partners/publications/pubdocs/DIY_Guide_May_2008.pdf