How to fire certain action before and after every ajax call

There are several ways to achieve what you are asking for. The only difference between them is how they are implemented, and it’s up to you to choose which method works best for your specific case. The methods also depend on which version of jQuery you are using, so I will split this answer into two sections.

For jQuery 1.5 and later

Adding multiple callbacks after init

Since jQuery 1.5 you can add multiple callbacks thanks to the overhauled API and newly introduced jqXHR object that is returned by .ajax. It implement the Promise (see Deferreds) interface, and we can use that to our advantage:

// fn to handle button-toggling
var toggleButton = function() {
    var button = $(".button");
    button.button(button.button("option", "disabled") ? "enable" : "disable");
}

// store returned jqXHR object in a var so we can reference to it
var req = $.ajax({
    beforeSend: toggleButton,
    success: function(msg){
        /* Do what you want */
    }
}).success(toggleButton);

// we can add even more callbacks
req.success(function(){ ... });

Using a prefilter

jQuery 1.5 also introduced prefilters which you can use to replace the global configuration of jQuery 1.4:

// good practice: don't repeat yourself, especially selectors
var button = $(".button");

$.ajaxPrefilter(function(options, _, jqXHR) {
    button.button("disable");
    jqXHR.complete(function() {
        button.button("enable");
    });
});

Note: The jqXHR section of the $.ajax entry has this notice about using jqXHR.success():

Deprecation Notice: The jqXHR.success(), jqXHR.error(), and
jqXHR.complete() callbacks are deprecated as of jQuery 1.8. To prepare
your code for their eventual removal, use jqXHR.done(), jqXHR.fail(),
and jqXHR.always() instead.

For jQuery 1.4 and earlier

Events and global configuration

Use .ajaxStart and .ajaxStop to bind callbacks to a specific selector. The events that trigger these callbacks will fire off on all Ajax requests.

$(".button").ajaxStart(function(){
    $(this).button("disable");
}).ajaxStop(function(){
    $(this).button("enable");
});

Use .ajaxSetup to setup a global ajax configuration. The settings object passed to .ajaxSetup will be applied to all Ajax requests, even those made by the shorthands .get, .getJSON and .post. Note that this isn’t recommended since it can easily clobber its functionality.

$.ajaxSetup({
    beforeSend: function(){
        $(".button").button("disable");
    },
    success: function(){
        $(".button").button("enable");
    }
});

Filter out requests in global event callbacks

If you need to filter out certain requests you can do that with.ajaxSend and .ajaxComplete where you can check the Ajax settings object. Something like this:

var button = $(".button");

// fn to handle filtering and button-toggling
var toggleButton = function(settings) {
    if (settings.url == "/specific/url")
        button.button(button.button("option", "disabled") ?
            "enable" : "disable");
    }
};

// bind handlers to the callbacks
button.ajaxSend(function(e,x,settings){
    toggleButton(settings);
}).ajaxComplete(function(e,x,settings){
    toggleButton(settings);
});

This can also be done with the previously mentioned .ajaxSetup by doing the same type of checks to the settings object that is passed to the callback handlers.