Unlocking an iPhone 4 for use in Europe

I would like to use my iPhone 4 in europe (France, Belgium, Spain, Italy, Germany) this spring, and would like some advice on how to proceed. I guess I would like to be able to swap in different sim cards depending on which country I am in.

There seems to be numerous sites that will 'jailbreak' or unlock my iphone, and I was hoping to save myself the headache and do it myself. Also, I don't know which sites are reputable.

Any pointers, articles, reputable sites or instructions would be greatly appreciated.

Thanks!