Going away for a few months at the start of October and have never done more than a 12 day holiday before. Getting somewhat prepared now (Visas etc) but haven't even thought about what to pack.

So question is - What is vital to pack?

I'll have clothes for a few different climates (from Spain in Oct to Russia in November), things like my ipod, kindle, universal plug adapter and the like, but what should any backpacker going away for a few months have in their 20 litre bag?

Anecdotes of travelling mishaps always welcomed.