My crank keeps undoing itself.

dogsballs [quote]Sam I'd take the crank off using a crank extractor (Halfords sell one for about a fiver if no-one has one to lend), degrease the BB spindle and the crank area where the spindle enters, do a light re-grease back on the spindle and reassemble.
Give it a week and retighten the crank bolt if required, then (touch wood) all should be well!

you should never lube a spindle!![/quote]

I'm in the 'apply a very thin wipe of grease to the spindle' school of thought. That's what my LBS did and that's what I do. It's a personal thing. Me and my lube.
I've never had cranks loosen/fall off (well once of my dad's bike but that was cottered and non-maintained).