Its very easy to chuck a few things in and then find your bag weighs a ton...I try to minimise now.
It all depends on how far you are riding. Tools are an obvious one, and small pump, perhaps a spare inner tube (or at least repair patches), I used to carry a DH tube but they are rather big n heavy so a lightweight one or patches will do.
Some drink in bladder, depending on how hot it is.
Gear cable, link to fix chain, lightweight waterproof jacket ?
Now Ive got my D30 back protector in back pack and iPhone attached to front (Product X) I aim to wear it more (sometimes I didnt bother) but try to keep it lighter.
Sometimes didnt bother when holidaying in Alps, but was always near-ish a bike shop (Morzine, Les Gets) and the very helpful Morzine Bike Mechanics van normally based at Les Linderets, who were very helpful in fixing my bike as can be seen in one of the gallery pix over there somewhere < ()On that occasion I had a backpack full of stuff but was missing a tool to get rear wheel off !
Ive got this one
http://www.ibackpacks.co.uk/Dakine-AC-N ... -p479.html (but paid a lot less than £100 !), about half that, it holds a lot of stuff, nice bladder compartment and the feature(s) I find very useful, it can carry trails helmet, full face and knee pads underneath.
Andy