Some baggage allowance websites:
What can't you take as hand luggage? This is rather hit and miss with regard to climbing equipment and often varies depending on how awake the security people are. The rule of thumb should be “If in doubt check it in.” BA publish a pretty good list of what you are not allowed which is more or less the same for all airlines.
However, with regard to climbing equipment, I would advise the following:
Don't take any of the following as hand luggage
What can I take as hand luggage so that my check-in luggage is light enough?
You have put all that iron mongery in your hold luggage and now it weights 6 tonnes. Why not put all your clothes, personal belongings, tent (less pegs and poles), sleeping bag, thermarest, spare shoes, toiletries, books, guides... anything that isn't dangerous in hand luggage? Make sure that your hand luggage bag isn't too big and if it is very heavy, practice carrying it in a way that doesn't make it look heavy, just in case someone suspects and wallops you with a hefty excess baggage bill. Don't forget the trusty duty free carrier bag. You can get at least 5kg of stuff in a sturdy carrier bag and stow it by your feet on board the plane. This is good for guidebooks and maps which you can read during the flight. You may also consider wearing your big boots on the plane, but you could also just wear some comfy shoes, tie your big boots together and throw them over your shoulder.
Reduce the weight of your rack
'Your rack is your castle' seems to be the typical British attitude. How many times have you gone climbing with your partner and both lugged a full rack to the bottom of the crag? Think ahead. If you are flying out with a climbing partner, you only need one rack between you. Think about where are you going and what you will be doing. You don't need that Camelot 4 if you are going to clip bolts in Spain. Split your rack evenly between both of you and try to split it in a way that won't restrict you too much if one bag goes missing for a day or two. You may also consider taking bits of kit that serve multiple uses. For example, you can create a quickdraw out of a prussuk loop and don't bother with those bulky ascenders and figure of eights for abseiling. An ATC and prussuk will do the same job for a quater of the weight.
How else can I reduce weight?
If you are flying out with a climbing partner, there is no need to take two bottles of shampoo or two tubes of toothpaste. Have a think about all the items you have both packed and then share. You may also compare the weight of similar pieces of gear you both own. Take the lightest stove for example.
I am at the airport and have to pay an excess
Despite your best efforts, you get to the airport and you are a few kilos over. The first tip would be to check in at the same time as whoever you are flying with. Most check-in assistants will take the total weight of all the bags in your party and as long as you are within the allowance on average, there is no problem. Alternatively, you could just pay the excess but that could be costly so the only solution is to either repack, or get rid of some items. Check out the tips above for packing hand luggage, but you may have to consider getting rid of some items. “But what do you get rid of?” I hear you cry in despair. Well have a think about it. It costs £5.50 per excess kilo on Ryanair but only about £1 to buy a new bottle of Shampoo. Can you see where I am going? Chuck out heavy disposable items which you can obtain abroad.
Last but not least
There is no point in arguing the toss over any decision against you. Airport workers do a difficult job in stressful conditions and don't have time to argue. However, a cheeky smile and a wise crack often go a mile and many a time I have been let through with a wink, a nod and a knowing “Don't do that again.”