I'd normally open up another browser window and go to my favourite online translator, which is fine, but I've found another, quicker way.
If you use Mozilla Firefox [and I heartilly reccomend it comparred to IE] you can download various Extentions/Plugins to customise your web-surfing experience.
'G Translate' https://addons.mozilla.org/firefox/918/
uses Google Translation services to translate various languages, Japanese being one of them. Once installed, you just highlight the text you want to convert to English, Right-Click then move down to "Translate" and it'll do the job live, without switching to another screen.
OK, from the example below, the translation isn't spot-on, but none of these services are


If you want to see what a whole page or paragraph looks like, you can open the results up in a new tab- it won't override the screen you're currently viewing.
The second benefit is being able to "Search Web For [your highlighted selection]".
Perfect for discovering a little more about that obscure series that may have native fansites and an official Japanese webpage.
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Another useful tool is 'Rikaichan' https://addons.mozilla.org/firefox/2471/
A dictionary for Japanese to English/German/French/Russian. After selecting a Japanese word on a web page, hover over it with the mouse and a popup appears translating it for you. Again, useful if you want to find out what particular words mean and how sentances are constrcted etc.
This utility can be toggled along with GTranslate and other addons.