Remove Value Of Input Element Onfocus

I've got a default value of some text in an input element when a page loads.
What I'd like to do is have the value disappear when the user clicks in the
input field.

I've figured out I can use:

onclick="this.value=''"

This works fine, however if for some reason the user adds his own input,
then click somewhere else/clicks back, the text has once again been erased.

So I thought of a function like the following, however it does not work:

function removeinput(x) {

if (x == 'Enter items not on standard list here...') {

x = ''

}
}

onclick="removeinput(this.value);"

I've also tried placing this.value inside of single quotes to no avail.