Optimizing this Javascript code

I think this is the simplest way to write your code in optimised way

$(function(){
    $('#buscar').bind('focus change blur' , function(){
  //write ur codes here
});

Having the extra knowledge, thanks to @deceze's insight, I would recommend factoring out most of this logic into a single function. Essentially all these conditions are checking if the field is empty or not.

Also, I am adding a check for any sort of whitespace since a single " " is just as irrelevant as two spaces " ".

function isTextFieldEmpty(text, placeholder) {
    var onlyWhitespace = /^\s+$/;
    return (text == "" || onlyWhitespace.test(text) || text == placeholder);
}

Then you can reuse this function for all the events - focus, blur, change.

$(function() {

    // to avoid repeating this text everywhere..
    var placeholder = 'Buscar...';

    $('#buscar').focus(function() {
        if (isTextFieldEmpty(this.value, placeholder)) {
            $(this).val('');
        }
    });

    $('#buscar').blur(function() {
        if (isTextFieldEmpty(this.value, placeholder)) {
            $(this).val(placeholder);
        }
    });

    $('#buscar').change(function() {
        if (isTextFieldEmpty(this.value, placeholder)) {
            $(this).css({ color: '#999' });
        }
        else {
            $(this).css({ color: '#000' });
        }
    });

});

I would go for something like the code below. It isolates the functions you need (test for empty string, or test for no value or the placeholder text). Also, I don't believe the change function is the right place to change the color; rather it should be in the focus and blur functions where you also change the value.

Also, if you are simply looking for a placeholder method for an input field, there are many already made for you, where you set the text and the CSS style. Also, HTML 5 has a placeholder attribute for this purpose - see http://davidwalsh.name/html5-placeholder. Some of the better placeholder plugins even use HTML 5 and fall back on JS if it is not supported.

Anyway, here is the code:

$.fn.isValBlank = function() {
    return $.trim($(this).val()) == "";
};

$.fn.isInputBlank = function() {
    return (($(this).isValBlank() || ($(this).val() === 'Buscar...'));
};

$(function(){
    $('#buscar').focus(function(){
        var val = $(this).val();
        if ($(this).isInputBlank()) {
            $(this).val('');
            $(this).css({'color':'#000'});
        }
    });

    $('#buscar').blur(function(){
        if ($(this).isValBlank()) {
            $(this).css({'color':'#999'});
            $(this).val('Buscar...');
        }
    });
});

If it were the future and HTML5 were everywhere, or if everybody used only current versions of Safari, Chrome, and Firefox, then you could knock out most of that code and replace it with this:

<input name="something" id="buscar" placeholder="Buscar...">

But since, as of this writing, it's still the present, you can tighten things up a little like this:

function isEdited(el,placeholder){
  placeholder = typeof placeholder == 'undefined' ? 'Buscar...' : placeholder;
  return $.trim( $(el).val() ) === placeholder;
}

$('#buscar').focus(function(){
  if( isEdited(this) )
      $(this).val('');
}).change(function(){
  if( isEdited(this) )
      $(this).css({'color':'#999999'});
  else
      $(this).css({'color':'#000000'});
}).blur(function(){
    if( $.trim( $(this).val() ) )
       $(this).val('Buscar...');
});

If you need to do a lot of this, you might consider the jQuery Placeholder Plug-in. It uses the placeholder attribute, so it's HTML5-friendly.

Also,

HTML input elements have a defaultValue property accessible by JavaScript. When talking about placeholder text input values, you can use this to help make code more reusable. This isn't perfect by any means but...

$(function(){

    $("input[type='text']").focus(textInputFocusHandler).change(textInputChangeHandler).blur(textInputBlurHandler);

});

function hasUserValue(textInput) {
    return (textInput.value!=textInput.defaultValue && /^\s+$/.test(textInput.value)==false && textInput.value!=="");
}

function textInputFocusHandler(event) {
    if(!hasUserValue(event.target)) event.target.value = "";
}

function textInputChangeHandler(event) {
    if(!hasUserValue(event.target)) event.target.style.color = "#999";
    else event.target.style.color = "#000";
}

function textInputBlurHandler(event) {
    if(!hasUserValue(event.target)) event.target.value = event.target.defaultValue;
}

Quick optimization:

$(function(){
$('#buscar').focus(function(){
    var t = $(this);
        var v = $.trim(t.val());
 if(v === '' || 'Buscar...'){
    $(this).val('');
        }
}).change(function(){
    var t = $(this);
        var v = $.trim(t.val());
 (v === '' || 'Buscar...' ? t.css({'color':'#999999'}) : t.css({'color':'#000000'}))
}).blur(function(){
    var t = $(this);
        var v = $.trim(t.val());
    if(v === '')
       t.val('Buscar...');
});
});