What to look for in an external HDD?

I will probably be buying an external HDD in the near future, and I wondered if anyone can recommend any brands that I should look out for, or any that I should avoid.

I'm planning to use it for stuff which is too important to delete, but not really important enough to make a proper backup of, so it won't actually get used all that often. It will spend most of the time switched off, and so transfer rate isn't really an issue. The issue for me is reliability. I'd like it to keep working for at least a reasonable amount of time.

I know it's not possible to guarantee which drives will be reliable, so what I'd like as much as anything is anecdotal evidence. If you've had a crappy external drive that quit on you after a couple of months, please let me know who makes it. If you had an excellent external drive that gave you a long troublefree service, tell me that about too. Hopefully with enough responses, I should be able to narrow the field to the decent manufacturers.

If anyone has retailer links in the UK to offer, then that would be just the cherry on the cake, but even if you're recommending something from the US, I might be able to find the same model over here, so go right ahead.

Jam it back in, in the dark.