Shouldn’t be a problem.  They just move people through pretty quick, so unless you have something on you that you shouldn’t, and you look like you have something on you that you shouldn’t, you should be fine.

The most confusing thing is arriving in the US — particularly for US citizens returning from abroad.  They hand out the necessary forms on the plane and there’s usually an explanation in the airline magazine — plus the flight crew will usually answer basic questions.  As long as you don’t buy tons of stuff in Europe, don’t have rare antiques, exotic animals, plants or other regulated items, and don’t bring in more than the allowed amounts of alcohol, tobacco, perfumes, jewelry, (most of that stuff is cheaper in the US, anyway) it’s not a problem.  It’s really pretty easy — although it seems complicated when you’re faced with the forms at the end of a mind-numbing flight.

Almost anywhere else, you just show your passport.

It’s really nothing to worry about. In 99% of places it’s as simple as walking through a door that is labeled green. Basically just keep following signs to the “Way Out” or the “Nothing to Declare” (unless of course you have something) and you should be OK.

Have a great trip.

Coming back into the US (Newark) from Cologne, the customs guy asked us where we had been. When we said Paris, we were sent to a secondary inspection. Because we had a one-hour connection, we did not need any more slowdowns, but that’s how it goes. Finally, we went through and the secondary guys barely looked at our stuff. I guess they had to consider contraband cheese or meat or something. Then we ran, re-checked our bags, went through security again, and made our connection as the plane was boarding. This is the first time I’ve had a secondary inspection anywhere from customs.

In US airports there are no “Nothing to Declare” signs/corridors, and it’s up to an officer who collects custom forms on exit to send anyone to “secondary inspection.” (Secondary inspection usually means they put your bags through what looks like a huge x-ray machine.)

There are just two things to remember. [ol][*]Very small fraction of passangers are sent to the secondary inspection, so you probably won’t be.
[*]If you are sent to secondary inspection don’t panic. They have nothing against you, so as long as you stay cool you’ll be through pretty quickly.
[/ol] 

Yeah, seriously, unless you are literally smuggling stuff, you’ll be alright. Everyone else has described it pretty well. They give you these scary forms on the airplane that talk about jailtime if you fill them out incorrectly, then you get to America and the guy doesn’t even look at it. I don’t remember all the details, but it’s pretty self explanitory what you do. I do remember we had this surly airport guard guy (kind of redundant, i know) who basically just asked where we were, where we were going, and if we had anything illegal. After kinda joking around (I think, haha) with us for a min, he was like, hmmmm alright, welcome to America, and let us thru.
 
When in doubt, just follow the crowd of people off the plane, chances are, SOMEONE knows what to do 

Nothing scary about the customs form…Canada requires you fill out a similar form. 
 
I have only been pulled aside once, and that was only because I actually declared some food (don’t remember what it was, but it is permitted, so needed to be declared).  Usually I don’t even bother to declare it.  They didn’t even open my bag, they just ask me what food I had and I told them, they send me right back out.  This was when I was flying back from Paris, and they had recently banned poultry product (I think shortly after the bird flu thing).  An older couple was already in the custom search area, the guy asked specifically if they had Foie Gras, and the couple didn’t even know what it was.
 
I do think they&nbsprofile people to search.  My mom has been stopped and bags seached by US customs several times when travelling into the US.  I guess 50ish chinese woman tend to smuggle in various prohibited food items, like dried calamari.