The first invaluable resource is CanooNet, an online grammar guide and inflection dictionary. I use Canoo every time I am using a verb in a strange setting, or need to double-check my inflection of it. The website is canoo.net, however I have also created a Mozilla Search Plugin (also should work on IE), that allows you to quickly search for a verb without going to the page. This plugin directs you to the inflection of the verb. If you have not entered a verb, or there is confusion, it will take you to a listing page to select what you actually want to look for. To install this plugin, go here.
The second also invaluable resource is LEO, an online dictionary. You can access it at dict.leo.org. To make the most out of this dictionary, it is important to click on the word you think is the correct entry, which automatically searches for that word and you can make sure it is the right thing for the context you are looking for. Also, a useful icon appears next to the search results, which you can mouse over. This displays a pronunciation sound-byte, a link to the CanooNet entry, and a link to the full dictionary entry. To learn more about search features on LEO, click here. To get a Firefox/IE search bar plugin for LEO, go here.