First of all, if that's the case (and not the phenomenons described in Thermostat Wars), it means that something is pretty much busted in your HVAC system.
Not necessarily the unit itself - ducts may leak, filter may be dirty (how many years ago did you change it last time?), you may have cut the trees on the southern side that provided you with shade and so on and so forth. Best bet would be to assess the overall system status - but that's going to cost you.
Next best assumption is - you don't have money not only to fix something if it is broken (1), but even to assess the status. Or, for some reason, this is not practical (say, the assessment, fix and/or upgrade is planned and budgeted, but the right time hasn't come yet). Well, this is not a nice situation to find yourself in, but not all hope is lost.
All the things below are pretty trivial in retrospect, but sometimes not obvious upfront, so I'm going to lay them out anyway.
First of all, set the thermostat to a lower temperature early in the morning and leave it there. Your house will store some energy and will help your A/C fight with the sun. You must realize this will somewhat increase your electric bill - but not much.
You probably heard that you can't run evaporative (a.k.a. swamp) cooler together with your other (known as refrigerating) A/C - the swamp cooler will be busy injecting humidity into the air, whereas refrigerating A/C will be busy removing it from there - thus adding $$$ to your electric bills and doing little else.
Also, be ready that the house will be significantly more difficult to cool some time after you stop using evaporative cooler - all that humidity it pumped in is still there, contained in the air, clothes, carpets, stuffed furniture and building frame.
Close all the blinds and shutters you have. If some of your windows don't have them, go buy some - they're relatively cheap and will significantly improve the insulation - not only they block the sun radiation, but also introduce insulating air pocket when closed.
Go get some insulation sheets from your favorite home supply store and stuff them between the shutters and windows - bad windows may contribute as high as a quarter of total heat load to the house.
Along the same lines, if you're planning to improve the house, don't go cheap on windows - get dual or triple pane tinted windows - and do your homework on selection. Makes the world of a difference - our upstairs bedroom on the south side of the house with good windows installed was significantly cooler than north facing rooms with old, single pane aluminum frame windows.
Make sure your ceiling fans work for you, not against you.
Well, that's just about it for starters...
In addition, there's a nice article about Tricks to Keep Your House Cool This Summer (except for the section about fans, which I disagree with) - and make sure you read the comments. Have to apply a good sanity filter to those, though...
(1) Having home warranty is relatively painless way of making sure this never happens; will cost you anywhere up from $250 a year, depending on the features - whether you have a pool, etc. - but it opens another can of worms altogether.