Most of the professionals I know use mental stops. They have an idea where they want to get out but like to watch the price action so they can use some discretion to see if they should actually get out at that point.
Sometimes price can trade at your stop level but it doesn't necessarily mean you should get out... it's hard to explain this to new traders because most of you can't see the order book so can't see exactly what is going on.
Let me try and explain.
Let's say you buy 3 lots of the British Pound future at 1.5000 as you believe there is support there and you decide to put a mental stop at 1.4990 (10 pips).
Now lets say that the person that is using the order book is seeing this:
1.5003 (6)
1.5002 (2)
1.5001 (1)
1.5000 (1)
(1) 1.4999
(4) 1.4998
(9) 1.4997
(5) 1.4996
(1) 1.4995
Suddenly, the market players pull their bids and offers away from the current price. Now we look like this:
1.5003 (1)
1.5002
1.5001
1.5000
1.4999
1.4998
1.4997
1.4996
1.4995
1.4994
1.4993
(1)1.4992
(1) 1.4991
(2) 1.4990
Now lets say that another player that is long suddenly needs to get out of a 4 lot position. The only option he has (even though the current price is still 1.5000) is to hit the 1 lot bid at 1.4992, the 1 bid at 1.4991 and the other 2 bid at 1.4990.
If one person decides that he really needs to get out and he hits those prices then the price can jump straight down to 1.4990 and stop you out and then if the offers stay where they are which often happens, someone may hit one of the offers and suddenly we are right back where we started at 1.5000 but you are out.
A person that was watching the order book calls this a "blip" and would most likely decide (after some cursing) that this is not a reason for getting out even though their stop has been theoretically hit.
Problem is, if you can't see this, you need to do the safe thing and exit.
Now one question I see a lot of new traders ask is: "why, just before something volatile like non-farm payrolls don't I place an entry 15 pips either side of the price and just see which one gets filled as price often moves sharply in that direction and I will get the mometum on my side".
The example above shows why this can fail dramatically.
If bids and offers are pulled well away from the market which is what happens just before a figure like this, then the people that need to transact regardless just hit the nearest bids and the offers and the price can blip down and then up as people get in or get out. The price trades but its not a reflection of the figure its a reflection of the reduced liquidity. Often you can get topped and tailed on these figures.