Some people love their dogs and let them behave as if they are human. Others know how important it is to draw the line; especially if you have a big dog.
Furniture access is a matter of some importance for two reasons: firstly, because it's mighty inconvenient to have to fight for space on your own couch; and secondly, because it strongly relates to the matter of dominance, which is of the utmost importance as far as a harmonious dog/owner relationship goes.
Let's face it; dogs are smart. They know that your furniture is your turf. He you allow your German Shepherd to jump on furniture whenever he wants then that's giving him a message. Think about it, do you jump onto your dogs furniture whenever you want? If you allow him to then what do you think he is thinking?
In general dogs should not be allowed on furniture until they are at least 6 months old. Those first months are an important part of training and development so you don't want to confuse the dog. If you allow your German Shepherd on the furniture at a young age, he'll see it as a right, not a privilege.
This is why, if you're going to allow your German Shepherd on the furniture, it's a fantastic idea to impose limits: to teach him that he can't just leap up as and when he chooses, but that he must wait for an invitation.
This is an easy task. All you have to do is pat the seat next to you, and - in a cheery, friendly tone - say, "Up you get!". Most dogs need little more encouragement than this, and will be up like a shot before the second syllable's even passed your lips.
You'll also need to enforce the "off" command - this allows you to relax in the knowledge that, when you want some leg room, it's there for the taking; and also reminds your dog, in no uncertain terms, that his furniture access is not a right - it's a privilege!
Just a note, it may be easier to get them to obey the "up you get" then the "off".
For more information on dog psychology and behavioral problems, check out
SitStayFetch. It's an absolute goldmine of valuable information and advice for the responsible dog-owner, and covers just about every topic you could ever need to raise a happy, healthy, well-adjusted dog - everything from obedience work to correcting problematic behaviors to dog-whispering to teaching 'tricks' is covered in full detail.