Leiden (or Leyden) is a city in The Netherlands. Drivers may not know what you mean if you pronounce this "lee-d'n", as the pronounciation is closer to "lie-d'n" or alternatively "lay-d'n".
Now this spot can be a bit tricky to get to, but it's the perfect spot for heading South and a great way for heading East. From the center it's about an 45 minutes to an hour's walk. From Train and bus station "Leiden Lammerschans" it should only be a about twenty minutes as follows:
The N11/A4 junction (Bottom right on map)
Head South East on Melchior Treublaan. Cross the Kanaalweg and onto the Kanaalbrug across the channel. Turn right onto Cronesteinpad, and -right afher you cross the rail crossing, take the first left into the park. It will soon split up into a bike path and a footpath, follow the bike path. (It's called "Boerenpad", though I'm not sure it's denoted onywhere) The path will lead to the other end of the park where a small connection on your left leads you up to where the A4 and N11 meet.
To hitch EAST to Utrecht hitch the hard shoulder of the road straight ahead (the N11) To hitch SOUTH to Rotterdam go right which leads to the A4.
Remember as always in The Netherlands, do not go past the signs that have the motorway or highway symbol on them, as you may get fined.
Note the reason Utrecht is listed under both East and North is that while the N11 (a local road) is the most direct way another common way of travelling to Utrecht is to pass over the South ring of Amsterdam, and so you can catch rides quite well this way too.
Plesmanlaan Shell station (left on map)
From the main exit (South East) of The Central trainstation head right (South West) on Schipholweg. Turn right onto Plesmanlaan after about 250 meters. This is the big tunnel, not the small one. Follow plesmanlaan for 1.5 km or so when a Shell station will appear on your right. Note that there is not a sidewalk the whole way, but you are allowed to walk until the petrol station, no problem.
From here, you're best off asking people if they pass by the what is commonly as the "bridge restaurant" on the A44, unless they go straight to your destination of course. This Service station with a bridge and a few restaurants is a great place from which to hitch on to the airport or Amsterdam.
Do not bother with the BP station on the left early on Plesmanlaan, as they are likely to send you away, depending on who is working that day.