The customs forms are easy, there are two of them... The immigration (migracion) one is taken when they stamp your passport, the guy barely looks at it other than to make sure the information matches that on your passport. For destination address usually the city is good enough. The other one is customs (aduanes), and the guys who take them at baggage claim only take about half of them and never actually look at them... Just food for thought when you're thinking of what to write on them. Immigration rarely has these forms available (like they should) or pens, so make sure you get the forms from your stewardess and get them filled out before you deplane.
As soon as you get off the plane, you have to go through immigration. The entrance is sort of in the middle of the terminal (so if you're at the west end of the airport you'll want to turn right, at the east end you'll turn left). Follow the signs that proudly say "Migracion" in spanish with "immigration" in tiny size 1 font underneath. There's no glassed off hallway like in american airports, you're in the terminal with everyone waiting to board. If you move quick you may beat the lineup, but in most cases it's about 20 minutes to get through immigration. The agents rarely talk to you or even look at you, and half of them don't really speak any english.
Once you go downstairs to baggage claim, there is a duty free store here. Even though stuff like cigarettes and alcohol are super cheap in Costa Rica, they are even cheaper here at the duty free, and they don't care if you're not leaving the country today. If you're a smoker, grab a carton for $10 and stick it in your bags, and smoke em while you're in town. Likewise for a bottle of your favorite scotch.
On your way out of the airport (walk towards "aduanes", ie. customs), you'll have to throw all your bags on a bigass xray machine, just like the normal airport security ones but 3x the size. There's no metal detector or anything, just a couple bored customs agents who are thankful to have a cushy government job and barely look at the screen. If one of them manages to get off his ass, hand him your customs form. I didn't actually have a customs form my last trip and just walked through.
Now usually you'll have made transportation arrangements before arriving. If you haven't, and this is your first time in the country, just spend the extra $3 and take an airport taxi. There are hundreds of guys hanging in front of the terminal (they're not allowed in the building) who will shout stuff at you in broken english like "ten dollars to san jose!", but you really are better off going with the official airport company. This is done when you are at the two sets of glass doors at the end of the airport, there'll be a bunch of guys in there with shirts and ties and they will just yell "taxi?" to you. Say "si" and give him your destination written down on a piece of paper. You'll prepay over at the counter, he'll lead you to your taxi and take you there. You don't owe the driver anything extra, and there is no tipping in Costa Rica, but if you're one of those people who gets off on it, throw him a buck or two.
That's about all I can think of... If you run into trouble in the airport, the phrase you want to remember (like anywhere else in latin america) is "Habla Inglise?" The airport is supposed to hire as many english speakers as possible, but sometimes their comprehension falls short (or they flat out lie to get the job). Enjoy your trip.
