In a clean the bar needs to reach a specific height - slightly below the navel - to allow the lifter to speed under it. After all pulling the bar higher takes more effort and strength. In competition it's better to pull a heavier weight at a low height than pulling a lighter weight high.
The problem comes when long arms put the bar too low. This causes the pull after extension to be longer and makes it difficult to get under the bar. In the recent post "Moment of Power" I mentioned the bar should be in or near the crease of the hip. Chances are if you clean with long arms the bar will be closer to your knees than your hips.
The easiest way to get the bar further up the thigh is by widening your grip on the bar. Spreading the hands apart automatically moves the bar up a bit. If the bar isn't high enough the next cue to use is to "sweep" the bar in after extension. Think of it almost like putting on pants really fast.
Technique aside, those two methods should take care of most lifters. If your arms reach down to your knees you'll still have trouble. Don McCauley suggested on the Pendlay forums having the shoulders shrugged before starting the lift. There's also pulling with bent arms, but I believe there's a certain finesse to doing this correct.
And of course you can always try this,
Looks easy doesn't it?
Your leverages might not favor Olympic weightlifting but don't let that discourage you.
Always experiment, persist, and strive to improve.