Buy reasonable amounts, say 3-5% every paycheck while it falls. Don't try to time the bottom. If here within the span of the next year you are buying *above* your cost basis, hold till it reaches double your cost basis then sell at 3-5% of your weekly paycheck's worth. Inevitably it'll fall in value again and then you can start accumulating again with a new cost-basis.
Good tactic, but personally I think "bottom" is around $300, if it still seems to be going down after that, then $250. After $250 though, just keep buying, as prices will never go down that low again xD