Clip over whole thing each summer. Maybe change the irrigation setting (if these are being watered during the summer).

Has the area become more shady over time? Junipers will definitely thin without strong light. If that is the case, shearing them will only weaken them.

I have a similar problem. We have a rocket juniper that is around 7 feet tall. It is dense and gets plenty of sunlight. The branches keep "hanging out" instead of keeping that tight rocket shape. My husband tied rope around them in hopes to train them back into shape, but when we untied it, the branches fell back down. It seems like they are too heavy. Do they just need to be pruned? Please post if you find any info.

OK... I ended up taking them down from 20ft to about 14 ft. And, since they weren't in increasing shade, I sheared them a little tighter than usual. While I took away the highest 1/3, I didn't cut it straight across the top. I left the branches that were higher than, and covering the new top.

It's over 3 years now, and I am happy with the results.