Know Where the Nutrition Is

It may or may not be common knowledge, but when it comes to most vegetables there is a large amount of nutrients locked up in the peel. So it just makes sense to juice your vegetables with the skins or peel still on. Obviously you have to make sure that the vegetables are thoroughly washed and scrubbed of any dirt beforehand to avoid getting any grit into your juice.
I know that some vegetables really do need to be peeled mainly because the skins are very tough and woody. Ginger, for example, has a very fibrous tough peel and in truth you only really want the flavor of the yellow flesh inside. If you have a decent juicer, however, it should be able to crush and juice tough skins so make sure you are not missing out on the best part of your vegetables.