I did this way twice,last time 2 weeks ago from this moment and it always worked.
To get out of the Prague direction north to the Germany,you should take the metro to Letnany - the line C (red line) and then try to take anything what is going to Čakovice (especially the bus going to Cakovice Shopping Center would be great option). From there is going just one Bus to Ďáblice (little devils place ;)
where you can enter on highway. But walking distance is about 200meters from this shopping center. As the bus goes every 40 minutes its easier and better to walk down there. When u will be approaching to highway - you will see that the traffic is getting very dense. I would recommend you rather to go to gasstation,as trying to catch someone directly on the exit to the highway is a bit complicated (even though there is one place where they can stop,they simply wouldnt do that).
And I found my ride on gasstation in 5 minutes. As there are comming as well the people for MC Donalds,there are lot of people stopping to get some meal ;)
I used this way twice and always was very successful and I have never waited longer than few minutes.
From this place this time I went directly to Usti nad Labem,but you can be lucky and find someone going directly to Berlin,Leipzing or anywhere you want to go. Just look on the number plates and make your pick ů0