This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
In general, the availability of bike parts varies greatly from bike shop to
bike shop. The larger European cities contain well stocked shops, however the
smaller towns(as you find in the mountains) are not as well stocked and parts
may be hard to find. This goes for service as well. It is a good idea to come
prepared to be able to deal with about anything, or have a bike that is low
maintenance (sealed components).
Here is my pre-tour bike preparation:
1) new chain
2) new tires and tubes
3) 4 new cables(2 - brake, 2 - derailleur, esp. if STI)
4) repack or replace bottom bracket
5) repack or replace headset
6) repack hubs
7) clean derailleurs
8) check brake pads for wear
9) true wheels
10) oil/grease freewheel/freehub
11) wash bike thoroughly(check frame for any cracks!)
I would recommend soldering the ends of your brake and derailleur cables. This
keeps the cables from fraying and you can take them in and out of their fitting
and housing when packing and unpacking the bike or doing maintenance and you
don't have to worry about losing those silly little aluminum end caps!