Just to make it clear, the limit for suitcases is 20kg, not 25kg unless you get a charter flight which they allow you more on.
As others have mentioned, the real trick is just taking a look at what you packed and asking yourself "do I really need that?"
Me and sam only went for a week so keeping it under 20kg was quite easy. Last year going to Gran Canaria we packed loads and came back having not worn half of what we packed, so this time we had a good think and realised that the week would only consist of swin costumes and a few pairs of shirts and tshirts, plus a few "smart" clothes. Dont forget that if you pack light and need to wear the same clothes again you can usually find a laundrette at your hotel or close by.
As for hand luggage, most people do not make the full use of this as it is another 5kg that you can take with you, and neither birmingham intl or ibiza airport weighed our hand luggage going in or out!
We also asked the same question regarding cd players and got quite vague and varied answers about it so decided to risk it. We took the cd player seperate to our hand luggage and carried it onto the plane with us, storing it over head next to the hand luggage. This also was not weighed and the only time anyone paid any attention to it was when we had to put it on the xray machine.
If you do go over then most people will let you off with a couple of kilos, but as K said... BE NICE! If you go over by more than that then do expect to have to put your hand in your pocket, and dont expect much change!
Just a quick note, if you find yourself going over on weight because of alcohol that you buy out there then consider the pre-order on your outgoing plane. I pre-ordered about 6 bottles of spirits and just picked them up from ibiza airport on the way home. This also was not weighed and we just took it on with our hand luggage as well as the cd player.
Its odd how you can seem to take so much on in the way of hand luggage and cd players, yet they get so awkward over suitcases. Still, if there is a way round these things then you may as well make the most of it.
Hope this helps guys, have a great time!