Hitchhiking isn't that popular for long distance rides within the local population. However, it works pretty well, especially close to beaches and in remote areas where there is almost no public transport. For hitchhiking the Panamerican highway you might wait a bit more then on the beaches but it's still not bad. Except on remote beaches, the popular hitchhiking with pick-up trucks doesn't work in Costa Rica and is illegal - the police does give high fines for violations of this particular law.
"Man, I had a blast hitching Costa Rica. Easy as blackberry-crumble. If you're lucky enough to cross mount Cedro del los Muertos without too much fog, you can see both the Atlantic and the Pacific oceans at once (or so they say). Don't get leichmaniasis!" - User:Kalan
You can get simple roadmaps with gas stations for free in the tourist information in San José. Other recommended maps: Reise Know-How "Costa Rica/Panama" scale: 1:550.000 (world mapping project). There are various maps in the Libreria Lehman in the pedestrian zone (Calle Central, in San José).
Fabzgy hitchhiking on the Pacific coast of Costa Rica