The act of picking a four-leaf clover once it has been found is an, often unquestioned, end to many people's approach to searching. I believe that this violent act is both inappropriate and unnecessary. The urge to pick a four-leaf clover can be traced to two related causes; the belief in the power of the four-leaf clover to bestow luck combined with a desire to have a physical token to represent the effort put forth in searching.
For me, the luck in finding a four-leaf clover is contained within the act of finding itself. You are lucky because you are able to take time out of your day to leisurely sift through clovers. You are lucky to be able to set aside your worries and focus your mind on something simple and relaxing. If you find a four-leaf clover when you are not actively looking for it (many people consider accidentally found four-leaf clovers to be the luckiest of all) the luck is still not a product of the clover itself which has always been there, you create your own luck by choosing to cross paths with it and by being aware enough of your surroundings to notice it.
The desire to have physical reminder of our efforts and accomplishments is a powerful human impulse. Often the reason we keep tokens is to help ourselves remember, we use an object as a shortcut to represent the memory of a whole event. As these tokens build up, however, they each become less evocative until we are often left with a multitude of trinkets which, because they only directly represented a small part of a larger memory, have little or no meaning left in them. One might be able to look at one pressed clover and fondly recall the events that surrounded finding it, but dozens of clovers do not offer the same result, vivid memory of dozens of events, instead you are left with a mere collection removed from any sentimental value. I find that keeping a record of the physical location where the clover was found, and the memory formed, is a much more powerful tool for remembering as it allows you to return your body as well as your mind to what you are trying to recall. Because a collection of picked clovers serves no sentimental value they are instead kept as a record of achievement. Without a record of our accomplishments we fear that all of our efforts will have been in vain. Picking clovers, however, is a very poor way of preserving the accomplishment of finding it permanently. Once removed from the nurturing safety of the earth clovers decay rapidly, even when pressed they will disintegrate and fade. A digital photo can potentially last indefinitely.
In the end it is a question of the respect you hold for the earth. If you are amazed by the wonder that the earth creates you should express it through reverence and not self-interest.
Please, do not pick four-leaf clovers.