Renting a car can be expensive, but there are ways to make it a little less painful on your pocket:
Become part of a loyalty scheme, ie Ryanair/Aer Lingus are linked with Hertz. Both have loyalty schemes and are linked together.
Don’t buy extra insurance, see if you are already covered on your own insurance.
Make sure the excess (the amount you will have to pay before insurance kicks in) is an acceptable amount for you.
Get a smaller car. They are cheaper to rent and cheaper to run.
Bring your own sat nav, or even better use maps like in the old days.
Join a car sharing club such as Zipcar or City Car Club, which charge by the hour, rather than by the day.
Remember, if they give you a full tank of gas, they want a full tank back.
Get an international driving license before you leave. Make sure you have breakdown cover as well.
If driving through Europe or anywhere in your own car, make sure you have your log book or proof of ownership with you.