Investment approaches

Method 1:
Lump-sum investing.

• How - Buy when stock at its lowest; Sell when its highest
  
• Pro - higher returns
• Con - higher risk; timing the market

Method 2:
Dollar-cost averaging.

• How - Invest same amount of money on a regular basis
• Pro - spread risk
  
• Con - averaging returns

Method 3:
Value-averaging.

• How - Investing more when market is low and less when market is high; on the basis of a fixed amount of value portfolio u wish to attain. For example, u wish to maintain a value of $1000 each month. Some months market is low, say your portfolio value down by $250, u will invest $1,250 to purchase more stock to maintain $1,000. While some months market is high, say your portfolio value is up $400, u will only need to invest $600 more for that month.
• Pro - more market sensitive and sensible in investment approach
• Con - cant think of any now.. perhaps doesn't provide the highest returns
  

Summary: For good net results, i believe value-averaging make a lot of sense.

disclaimer: author isn't giving any professional advise. just storing info he gotten from some other smarter author.