Defining #default_value for a select form item

I have an array where there are some options already loaded (it's a nodereference field that allows referencing only the nodes that are from one content type).

If I print_r($form['field_recurso']); (which is the field containing the select box I want to modify), I get

Array ( [#tree] => 1 [nids] => Array ( [#type] => select [#title] => recurso [#default_value] => Array ( [0] => 0 ) [#multiple] => 0 [#size] => 0 [#options] => Array ( [417] => Canón de vídeo NEC [446] => Canón de vídeo PLUS [447] => Portátil PC [418] => Sala de Xuntas ) [#required] => 1 [#description] => [#post] => Array ( ) [#programmed] => [#tree] => 1 [#parents] => Array ( [0] => field_resource [1] => nids ) [#weight] => 0 [#processed] => [#attributes] => Array ( ) [#input] => 1 [#name] => field_resource[nids] [#id] => edit-field-resource-nids [#value] => Array ( [0] => 0 ) [#sorted] => 1 ) [#weight] => -10 [#post] => Array ( ) [#programmed] => [#parents] => Array ( [0] => field_resource ) [#processed] => [#sorted] => 1 ) 

I tried several options to change default value, and the value of the array item changes, but it makes no effect.
I tried setting #default_value to a integer (418, 447, the nids of the referenced nodes), to an array (containing the nid and the node title: arrray(418=>Sala de Xuntas), to the node title (Sala de Xuntas)... but I had no luck.

Any clue?

For reference, my whole form looks like this:

<div id="resource"><?php print drupal_render($form['field_resource']); ?></div>
<div id="start"><?php print drupal_render($form['start_date']); ?></div>
<div id="end"><?php print drupal_render($form['end_date']); ?></div>
<div id="description"><?php print drupal_render($form['field_description_0']); ?></div>
<?php 
print drupal_render($form['form_token']);
print drupal_render($form['form_id']);

// print drupal_render($form['preview']);
print drupal_render($form['submit']);
print drupal_render($form['delete']);
?>