django-grappelli provides this as one of the features. Here s the wiki page about that feature (with screenshots).

admin.py:

class PageAdmin(admin.ModelAdmin):
    fieldsets = (
        (None, {
             fields : ( title ,  content , )
        }),
        ( Other Informations , {
             classes : ( collapse ,  open ),
             fields : ( slug ,  create-date ,)
        }),
    )

templates/app_label/model_name/change_form.html:

{% extends "admin/model_name/change_form.html" %}

{% block extrahead %}
    {{ block.super }}
    <script src="{{ STATIC_URL }}admin/js/collapse-open.js" type="text/javascript"></script>
{% endblock %}

static/admin/js/collapse-open.js:

(function($) {
    $(document).ready(function() {
        $( fieldset.collapse.open ).removeClass( collapsed );
    });
})(django.jQuery);

I know this is real old but I also just came across this issue. After thinking about it way too hard I found an easy solution which seems to get the job done involving 0 plugins or additional js.

Within fieldsets construct Add collapse in rather than collapse to class:

fieldsets = [
    ( Start Expanded , {
     fields : [ field1 ,  field2 ,  field3 ], 
     classes : [ collapse in ,]
    })
]

Accoding with the grappelli docs only need to add "classes" : ( grp-collapse grp-closed )

for example

class EntryOptions(admin.ModelAdmin):
   ...
   fieldsets = (
     (  , {
         fields : ( title ,  subtitle ,  slug ,  pub_date ,  status ,),
     }),
     ( Flags , {
         classes : ( grp-collapse grp-closed ,),
         fields  : ( flag_front ,  flag_sticky ,  flag_allow_comments ,),
    }),

note: if you use grappelli version < 2.4 use grp-closed instead collapse-closed * actually collapse-close still is working but is recommended to use the new classes

Starting from Setomidor answer, I d like to suggest a simpler alternative that does exactly what you want (if Grappelli is not an option, obviously).

Create the template override as explained (admin/(app)/(model)/change_form.html) and instead of removing the collapsible effect for the "add" model case, simply call the click method of the field-set collapser (i.e. the small link with the show/hide text) to have the whole field-set expanded as soon as the page loads.

The quickest hack I could find was to add a new class start-open to the fieldset

classes = [ collapse ,  start-open ]

and then modify static/admin/js/collapse.js.

from:

    // Add toggle to anchor tag
    var toggles = document.querySelectorAll( fieldset.collapse a.collapse-toggle );
    var toggleFunc = function(ev) {
        ev.preventDefault();
        var fieldset = closestElem(this,  fieldset );
        if (fieldset.classList.contains( collapsed )) {
            // Show
            this.textContent = gettext( Hide );
            fieldset.classList.remove( collapsed );
        } else {
            // Hide
            this.textContent = gettext( Show );
            fieldset.classList.add( collapsed );
        }
    };
    for (i = 0; i < toggles.length; i++) {
        toggles[i].addEventListener( click , toggleFunc);
    }

to:

    // Add toggle to anchor tag
    var toggles = document.querySelectorAll( fieldset.collapse a.collapse-toggle );
    // NEW: select toggles to open
    var open_toggles = document.querySelectorAll( fieldset.collapse.start-open a.collapse-toggle );
    var toggleFunc = function(ev) {
        ev.preventDefault();
        var fieldset = closestElem(this,  fieldset );
        if (fieldset.classList.contains( collapsed )) {
            // Show
            this.textContent = gettext( Hide );
            fieldset.classList.remove( collapsed );
        } else {
            // Hide
            this.textContent = gettext( Show );
            fieldset.classList.add( collapsed );
        }
    };
    for (i = 0; i < toggles.length; i++) {
        toggles[i].addEventListener( click , toggleFunc);
    }
    // NEW: open toggles
    for (i = 0; i < open_toggles.length; i++) {
        toggles[i].click();
    }

Old question, but I ran into the same one and came up with an answer which can be implemented using standard django:

create file: admin/(app)/(model)/change_form.html in your templates directory to make your (model) of your (app) use that form file.

Add this code to the file:

{% extends "admin/change_form.html" %}

{% block extrahead %}
    <!-- Load superblock (where django.jquery resides) -->
    {{ block.super }}
    <!-- only do this on  add  actions (and not  change  actions) -->
    {% if add and adminform %}
    <script type="text/javascript">
        (function($) {
            $(document).ready(function($) {
                //Remove the  collapsed  class to make the fieldset open
                $( .collapse ).removeClass( collapsed );

                //Remove the show/hide links
                $( .collapse h2 a ).remove();

                //Tidy up by removing the parenthesis from all headings
                var $headings = $(".collapse h2");
                $headings.each(function(i, current){
                    var str = $(current).text();
                    parenthesisStart = str.indexOf( ( );
                    $(current).text(str.substring(0, parenthesisStart));
                });
            });                                 
        })(django.jQuery);                      
    </script>
    {% endif %}
{% endblock %}

For more information about using django.jQuery instead of "regular" jQuery, see: http://blog.picante.co.nz/post/Customizing-Django-Admin-with-jQuery--Getting--is-not-a-function/

So this one worked for me:

class PageAdmin(admin.ModelAdmin):
    fieldsets = (
        (None, {
             fields : ( title ,  content , )
        }),
        ( Other Informations , {
             classes : ( wide ),
             fields : ( slug ,  create-date ,)
        }),
    )

With django 4.x, here is my admin/js/collapse.js add start-open in class

 use strict ;
{
    window.addEventListener( load , function() {
        // Add anchor tag for Show/Hide link
        const fieldsets = document.querySelectorAll( fieldset.collapse );
        // NEW: select toggles to open
        var open_toggles = document.querySelectorAll( fieldset.collapse.start-open );        
        for (const [i, elem] of fieldsets.entries()) {
            // Don t hide if fields in this fieldset have errors
            if (elem.querySelectorAll( div.errors, ul.errorlist ).length === 0) {
                if (elem.classList[3] !=  start-open ) {
                    elem.classList.add( collapsed );
                }
                const h2 = elem.querySelector( h2 );
                const link = document.createElement( a );
                link.id =  fieldsetcollapser  + i;
                link.className =  collapse-toggle ;
                link.href =  # ;
                if (elem.classList[3] !=  start-open ) {
                    link.textContent = gettext( Show );
                } else {
                    link.textContent = gettext( Hide );
                }
                h2.appendChild(document.createTextNode(  ( ));
                h2.appendChild(link);
                h2.appendChild(document.createTextNode( ) ));
            }
        }
        // Add toggle to hide/show anchor tag
        const toggleFunc = function(ev) {
            if (ev.target.matches( .collapse-toggle )) {
                ev.preventDefault();
                ev.stopPropagation();
                const fieldset = ev.target.closest( fieldset );
                if (fieldset.classList.contains( collapsed )) {
                    // Show
                    ev.target.textContent = gettext( Hide );
                    fieldset.classList.remove( collapsed );
                } else {
                    // Hide
                    ev.target.textContent = gettext( Show );
                    fieldset.classList.add( collapsed );
                }
            }
        };
        document.querySelectorAll( fieldset.module ).forEach(function(el) {
            el.addEventListener( click , toggleFunc);
        });
    });
}