the best way would be to define an envelope for the specific sample, by clicking:
sample channel > channel settings > INS > VOL, and then tweaking the knobs in the Envelope sub-window like this:
(max left = 0 , max right = 255)
DEL 0
ATT 0
HOLD 255
DEC 0
SUS 255
REL 0
now, what you've just programmed is an envelope that effects the sample's volume.
keep in mind that a sample played in the Step Sequencer (the buttons next to the channel's name) will not adjust to these settings.
only samples played via the Piano Roll will be affected by the envelope. (you bring up the Piano Roll by hitting F5 on the selected channel or by opening the PR button on the topmost right panel of the screen).
therefore, your best bet would be to use ALWAYS the Piano Roll, unless the sample you want to play is a percussive sample (i.e. a short sample, like a drumhit or a sound effect). if you use the Step Sequencer on the sample, every time there is a note, it will trigger the sample and play it until the end... this can be counter-affected by right-clicking on the channel and selecting "cut itself". however, this method has the downside of playing the sample until its end.
if what you want to do is to STOP the sample at a specific point, you have to use the Piano Roll to enter it (the sample will stop playing at the point the note in the Piano Roll ends).
this is because the Piano Roll adds the Length function to each of the notes it plays. therefore a loop will be more controllable when it comes to its duration using this method.
poster, if you feel you need a little more help with this technique, by all means contact me. i'm a fruity loops enthusiast who has mastered all of its tricks. even though i currently don't have a 24/7 availability, i will be happy to provide you with further help.