If you are interested in ETF's then they work the same as buying a single stock. The following money management tool is applicable as well.
Think of it in terms of:
Good Plan + Good Outcome = Deserved Success
Good Plan + Poor Outcome = Bad Luck
Bad Plan + Good Outcome = Dumb Luck
Bad Plan + Poor Outcome = Poetic Justice
The key to cutting losses and running profits in Shares/ETF's is the trailing stop loss.
For example, you have €128,000 capital.
You want to only risk 1% of your capital on a trade = €1,280.
i.e. If you buy the shares today and your stop loss is hit, then you only lose 1% of your capital.
This is opposed to simply saying "I'm putting 15k in Paddy Power"
An Example.
You would like to buy Paddy Power shares trading at €29.48 on Friday 4th March 2011.
You will use a 25% stop loss = sell at €22.11 (don't even think about it, just have the discipline to get out)
The price needs to drop 25% before you admit defeat and sell at a loss.
Using the following calculation, You can determine how many shares to buy
Google 'Position sizing calculator'
So You will buy 174 PP shares at 29.48 = €5,129 investment.
Should the price fall and go to €22.11, You will sell
22.11 * 174 = €3,847
You will only lose 1% of your capital. You still have €126,720 overall.
Now with the trailing stop loss, should the price of Paddy Power rise, You will adjust the stop loss UPWARDS each time the price increases (reaches a new high).
E.G. On Friday 18th March, the price is €30.30
Your 25% stop loss is now €22.73
Should the price now fall (you would make less of a loss), You leave the stop loss as it is. Only ever increasing the stop with an overall increase in price.
This is a long term strategy where shares are held and any dividends are re-invested.
Before you get in, know when you'll get out!
Hope this gives you an idea of money management.