Jeroen,
Nice tree. as mentioned in one of the previous posts: What are your idea with respect to size? Will it stay this size?
Assuming it will: Next year, try to keep all the branches that you would like to keep short. Long enough to get light, but don't let them grow too much. Rather, give 2 or 3 branches the opportunity to shoot out. This will create bulk for the trunk, creating a more aged look very quickly. Once you have decided the main stem is thick enough, remove the branches. The stumb you could turn into deadwood, or you remove them flush with the main stem. You could repeat this a few years in a row. As long as you let the remaining branches grow a little, and ensure light reaches them, you should be fine.
I find that you can trim junipers quite well. I removed a lot of foliage in early summer, all the way back to the last cm of green needles & green stem. These cuts produced massive branching around the cut, which I then thinned. You can do strimming on Juniperus when they are in active strong growth, untill September, considering a northern European climate (Winter with minus temperatures from mid october or so). After early September the tree is left to heal whatever wounds are there, stoch up on some nutrients and get ready for winter.
Considering your name, I assume you are Dutch, and understand the language. Have a look at my posts at the Dutch forum:
www.bonsaiempire.nl/forum/vormingen/6286...us-communis?start=10 . Note that this is a different species though (!)