If the market plummets 50% tomorrow, should I buy some more stock?
Or, is it better to have put that money in the year before even if it was more expensive at the time?[/b]
You should by stock according to your pre-determined plan. If something in that plan triggers a buy tomorrow or next week, then you should continue to do it. If it doesn't, you shouldn't.
As to whether it is better to pave put that money in a year ago, in theory, the depends on the specific numbers, including dividends. But the expense of designing, building, and maintaining a time machine has to be subtracted from the increased gains. Therefore, you can only look forward.
So, the answer is to come up with an investment plan (Personal Investing Statement, whatever) in a cold, unemotional way. Then implement that plan with no regard for what the market did last week, last year, or yesterday, or with what you think you might know about what it will do tomorrow or next week or next year.
It seems like most people agree that market-timing is ineffective and generally bad. But they are unable to actually implement that when it comes to executing an actual plan.