Be wary of groups of young men who try to carry your bag/open taxi doors etc for you as soon as you come out of the airport, then demand a 'trinket' - usually currency. It can be quite dazing and confusing after a long haul flight, and a long time waiting in a queue for a visa - I thought the guy who carried my bag was from the 'pick up' guy from the hotel I was staying in, as I wasn't quite as on the ball as I usually am!
When we were checking in for our flight back to Delhi, there were a few uniformed guys at the baggage counters, openly demanding a bribe to let baggage thru without paying excess baggage charges. Also, to help you jump the queue etc...
Infact, we had forgotten to X-ray our bag since we we thought we were running late. The guys at check-in didnt seem to think it was a problem, and checked in our bags without even a X-ray screening!
A few weeks later, when the same Delhi-bound flight was hijacked to Kandahar, in the now infamous episode, i was not surprised that the hijackers chose to board at Kathmandu...
Fill up your travel visa (with photo attached) beforehand and get ready exactly US$30 for the visa fees when you arrive at the Tribuvan Airport, so that they could process your visa. This will save you a lot of time especially if you are landing late at night, trust me.
Have a plan before you walk out of the airport in Kathmandu. You will be astounded at the mob of people shouting and trying to take your bag to a cab. It isn't aggressive in a rough way but it was definitely intimidating. It was actually fun to stand back and watch as each person leaving the airport was overtaken by the mob. Also, beware of people who follow your cabbie and want to carry your bag. They are not with him and will expect to be tipped. If you have a lot of stuff it might be worth it but know that they will expect money.
When you arrive at the airport there will probably be a swarming crowd of people and can feel a bit intimidating. Try to have something arranged prior to your arrival such as transportation so you don't get sucked into an overpriced cab ride or something worse. A few fellows offered to carry my luggage to the van ( I stupidly thought out of the kindness of their hearts! ) and then angrily demanded rupees so be warned!
When leaving the airport have all your lugage in a firm grip. You will be stormed by men who want to carry your suitcases and backpacks to any expensive taxi they see, and expect a rather large fee for this 'service'. Be rude to them if that is the only way to get rid of them!
Even if you are not a good negotiator don't pay more than 200 rupies for a taxi-ride to a Kathmandu hotel. If haggle is your sport you should be able to get it for 100 rupies!